[PATCH v2 0/4] mm/memcg: Address PREEMPT_RT problems instead of disabling it.

[PATCH v2 0/4] mm/memcg: Address PREEMPT_RT problems instead of disabling it.

[Sebastian Andrzej Siewior]

Hi,

this series aims to address the memcg related problem on PREEMPT_RT.

I tested them on CONFIG_PREEMPT and CONFIG_PREEMPT_RT with the
tools/testing/selftests/cgroup/* tests and I haven't observed any
regressions (other than the lockdep report that is already there).

Changes since v1:
- Made a full patch from Michal Hocko's diff to disable the from-IRQ vs
  from-task optimisation

- Disabling threshold event handlers is using now IS_ENABLED(PREEMPT_RT)
  instead of #ifdef. The outcome is the same but there is no need to
  shuffle the code around.

v1: https://lore.kernel.org/all/20220125164337.2071854-1-bigeasy@linutronix.de/

Changes since the RFC:
- cgroup.event_control / memory.soft_limit_in_bytes is disabled on
  PREEMPT_RT. It is a deprecated v1 feature. Fixing the signal path is
  not worth it.

- The updates to per-CPU counters are usually synchronised by disabling
  interrupts. There are a few spots where assumption about disabled
  interrupts are not true on PREEMPT_RT and therefore preemption is
  disabled. This is okay since the counter are never written from
  in_irq() context.

RFC: https://lore.kernel.org/all/20211222114111.2206248-1-bigeasy@linutronix.de/

Sebastian

[PATCH v2 0/4] mm/memcg: Address PREEMPT_RT problems instead of disabling it.

[Sebastian Andrzej Siewior]

Hi,

this series aims to address the memcg related problem on PREEMPT_RT.

I tested them on CONFIG_PREEMPT and CONFIG_PREEMPT_RT with the
tools/testing/selftests/cgroup/* tests and I haven't observed any
regressions (other than the lockdep report that is already there).

Changes since v1:
- Made a full patch from Michal Hocko's diff to disable the from-IRQ vs
  from-task optimisation

- Disabling threshold event handlers is using now IS_ENABLED(PREEMPT_RT)
  instead of #ifdef. The outcome is the same but there is no need to
  shuffle the code around.

v1: https://lore.kernel.org/all/20220125164337.2071854-1-bigeasy@linutronix.de/

Changes since the RFC:
- cgroup.event_control / memory.soft_limit_in_bytes is disabled on
  PREEMPT_RT. It is a deprecated v1 feature. Fixing the signal path is
  not worth it.

- The updates to per-CPU counters are usually synchronised by disabling
  interrupts. There are a few spots where assumption about disabled
  interrupts are not true on PREEMPT_RT and therefore preemption is
  disabled. This is okay since the counter are never written from
  in_irq() context.

RFC: https://lore.kernel.org/all/20211222114111.2206248-1-bigeasy@linutronix.de/

Sebastian

[PATCH v2 1/4] mm/memcg: Revert ("mm/memcg: optimize user context object stock access")

[Sebastian Andrzej Siewior]

From: Michal Hocko <mhocko@suse.com>

The optimisation is based on a micro benchmark where local_irq_save() is
more expensive than a preempt_disable(). There is no evidence that it is
visible in a real-world workload and there are CPUs where the opposite is
true (local_irq_save() is cheaper than preempt_disable()).

Based on micro benchmarks, the optimisation makes sense on PREEMPT_NONE
where preempt_disable() is optimized away. There is no improvement with
PREEMPT_DYNAMIC since the preemption counter is always available.

The optimization makes also the PREEMPT_RT integration more complicated
since most of the assumption are not true on PREEMPT_RT.

Revert the optimisation since it complicates the PREEMPT_RT integration
and the improvement is hardly visible.

[ bigeasy: Patch body around Michal's diff ]

Link: https://lore.kernel.org/all/YgOGkXXCrD%2F1k+p4@dhcp22.suse.cz
Link: https://lkml.kernel.org/r/YdX+INO9gQje6d0S@linutronix.de
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 mm/memcontrol.c | 94 ++++++++++++++-----------------------------------
 1 file changed, 27 insertions(+), 67 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 09d342c7cbd0d..4b1572ae990d8 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2085,23 +2085,17 @@ void unlock_page_memcg(struct page *page)
 	folio_memcg_unlock(page_folio(page));
 }
 
-struct obj_stock {
+struct memcg_stock_pcp {
+	struct mem_cgroup *cached; /* this never be root cgroup */
+	unsigned int nr_pages;
+
 #ifdef CONFIG_MEMCG_KMEM
 	struct obj_cgroup *cached_objcg;
 	struct pglist_data *cached_pgdat;
 	unsigned int nr_bytes;
 	int nr_slab_reclaimable_b;
 	int nr_slab_unreclaimable_b;
-#else
-	int dummy[0];
 #endif
-};
-
-struct memcg_stock_pcp {
-	struct mem_cgroup *cached; /* this never be root cgroup */
-	unsigned int nr_pages;
-	struct obj_stock task_obj;
-	struct obj_stock irq_obj;
 
 	struct work_struct work;
 	unsigned long flags;
@@ -2111,12 +2105,12 @@ static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
 static DEFINE_MUTEX(percpu_charge_mutex);
 
 #ifdef CONFIG_MEMCG_KMEM
-static void drain_obj_stock(struct obj_stock *stock);
+static void drain_obj_stock(struct memcg_stock_pcp *stock);
 static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 				     struct mem_cgroup *root_memcg);
 
 #else
-static inline void drain_obj_stock(struct obj_stock *stock)
+static inline void drain_obj_stock(struct memcg_stock_pcp *stock)
 {
 }
 static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
@@ -2193,9 +2187,7 @@ static void drain_local_stock(struct work_struct *dummy)
 	local_irq_save(flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
-	drain_obj_stock(&stock->irq_obj);
-	if (in_task())
-		drain_obj_stock(&stock->task_obj);
+	drain_obj_stock(stock);
 	drain_stock(stock);
 	clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);
 
@@ -2770,41 +2762,6 @@ static struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
  */
 #define OBJCGS_CLEAR_MASK	(__GFP_DMA | __GFP_RECLAIMABLE | __GFP_ACCOUNT)
 
-/*
- * Most kmem_cache_alloc() calls are from user context. The irq disable/enable
- * sequence used in this case to access content from object stock is slow.
- * To optimize for user context access, there are now two object stocks for
- * task context and interrupt context access respectively.
- *
- * The task context object stock can be accessed by disabling preemption only
- * which is cheap in non-preempt kernel. The interrupt context object stock
- * can only be accessed after disabling interrupt. User context code can
- * access interrupt object stock, but not vice versa.
- */
-static inline struct obj_stock *get_obj_stock(unsigned long *pflags)
-{
-	struct memcg_stock_pcp *stock;
-
-	if (likely(in_task())) {
-		*pflags = 0UL;
-		preempt_disable();
-		stock = this_cpu_ptr(&memcg_stock);
-		return &stock->task_obj;
-	}
-
-	local_irq_save(*pflags);
-	stock = this_cpu_ptr(&memcg_stock);
-	return &stock->irq_obj;
-}
-
-static inline void put_obj_stock(unsigned long flags)
-{
-	if (likely(in_task()))
-		preempt_enable();
-	else
-		local_irq_restore(flags);
-}
-
 /*
  * mod_objcg_mlstate() may be called with irq enabled, so
  * mod_memcg_lruvec_state() should be used.
@@ -3075,10 +3032,13 @@ void __memcg_kmem_uncharge_page(struct page *page, int order)
 void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
 		     enum node_stat_item idx, int nr)
 {
+	struct memcg_stock_pcp *stock;
 	unsigned long flags;
-	struct obj_stock *stock = get_obj_stock(&flags);
 	int *bytes;
 
+	local_irq_save(flags);
+	stock = this_cpu_ptr(&memcg_stock);
+
 	/*
 	 * Save vmstat data in stock and skip vmstat array update unless
 	 * accumulating over a page of vmstat data or when pgdat or idx
@@ -3129,26 +3089,29 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
 	if (nr)
 		mod_objcg_mlstate(objcg, pgdat, idx, nr);
 
-	put_obj_stock(flags);
+	local_irq_restore(flags);
 }
 
 static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
 {
+	struct memcg_stock_pcp *stock;
 	unsigned long flags;
-	struct obj_stock *stock = get_obj_stock(&flags);
 	bool ret = false;
 
+	local_irq_save(flags);
+
+	stock = this_cpu_ptr(&memcg_stock);
 	if (objcg == stock->cached_objcg && stock->nr_bytes >= nr_bytes) {
 		stock->nr_bytes -= nr_bytes;
 		ret = true;
 	}
 
-	put_obj_stock(flags);
+	local_irq_restore(flags);
 
 	return ret;
 }
 
-static void drain_obj_stock(struct obj_stock *stock)
+static void drain_obj_stock(struct memcg_stock_pcp *stock)
 {
 	struct obj_cgroup *old = stock->cached_objcg;
 
@@ -3204,13 +3167,8 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 {
 	struct mem_cgroup *memcg;
 
-	if (in_task() && stock->task_obj.cached_objcg) {
-		memcg = obj_cgroup_memcg(stock->task_obj.cached_objcg);
-		if (memcg && mem_cgroup_is_descendant(memcg, root_memcg))
-			return true;
-	}
-	if (stock->irq_obj.cached_objcg) {
-		memcg = obj_cgroup_memcg(stock->irq_obj.cached_objcg);
+	if (stock->cached_objcg) {
+		memcg = obj_cgroup_memcg(stock->cached_objcg);
 		if (memcg && mem_cgroup_is_descendant(memcg, root_memcg))
 			return true;
 	}
@@ -3221,10 +3179,13 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
 			     bool allow_uncharge)
 {
+	struct memcg_stock_pcp *stock;
 	unsigned long flags;
-	struct obj_stock *stock = get_obj_stock(&flags);
 	unsigned int nr_pages = 0;
 
+	local_irq_save(flags);
+
+	stock = this_cpu_ptr(&memcg_stock);
 	if (stock->cached_objcg != objcg) { /* reset if necessary */
 		drain_obj_stock(stock);
 		obj_cgroup_get(objcg);
@@ -3240,7 +3201,7 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
 		stock->nr_bytes &= (PAGE_SIZE - 1);
 	}
 
-	put_obj_stock(flags);
+	local_irq_restore(flags);
 
 	if (nr_pages)
 		obj_cgroup_uncharge_pages(objcg, nr_pages);
@@ -6821,7 +6782,6 @@ static void uncharge_folio(struct folio *folio, struct uncharge_gather *ug)
 	long nr_pages;
 	struct mem_cgroup *memcg;
 	struct obj_cgroup *objcg;
-	bool use_objcg = folio_memcg_kmem(folio);
 
 	VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
 
@@ -6830,7 +6790,7 @@ static void uncharge_folio(struct folio *folio, struct uncharge_gather *ug)
 	 * folio memcg or objcg at this point, we have fully
 	 * exclusive access to the folio.
 	 */
-	if (use_objcg) {
+	if (folio_memcg_kmem(folio)) {
 		objcg = __folio_objcg(folio);
 		/*
 		 * This get matches the put at the end of the function and
@@ -6858,7 +6818,7 @@ static void uncharge_folio(struct folio *folio, struct uncharge_gather *ug)
 
 	nr_pages = folio_nr_pages(folio);
 
-	if (use_objcg) {
+	if (folio_memcg_kmem(folio)) {
 		ug->nr_memory += nr_pages;
 		ug->nr_kmem += nr_pages;
 
-- 
2.34.1

[PATCH v2 1/4] mm/memcg: Revert ("mm/memcg: optimize user context object stock access")

[Sebastian Andrzej Siewior]

From: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>

The optimisation is based on a micro benchmark where local_irq_save() is
more expensive than a preempt_disable(). There is no evidence that it is
visible in a real-world workload and there are CPUs where the opposite is
true (local_irq_save() is cheaper than preempt_disable()).

Based on micro benchmarks, the optimisation makes sense on PREEMPT_NONE
where preempt_disable() is optimized away. There is no improvement with
PREEMPT_DYNAMIC since the preemption counter is always available.

The optimization makes also the PREEMPT_RT integration more complicated
since most of the assumption are not true on PREEMPT_RT.

Revert the optimisation since it complicates the PREEMPT_RT integration
and the improvement is hardly visible.

[ bigeasy: Patch body around Michal's diff ]

Link: https://lore.kernel.org/all/YgOGkXXCrD%2F1k+p4-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org
Link: https://lkml.kernel.org/r/YdX+INO9gQje6d0S-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org
Signed-off-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
---
 mm/memcontrol.c | 94 ++++++++++++++-----------------------------------
 1 file changed, 27 insertions(+), 67 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 09d342c7cbd0d..4b1572ae990d8 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2085,23 +2085,17 @@ void unlock_page_memcg(struct page *page)
 	folio_memcg_unlock(page_folio(page));
 }
 
-struct obj_stock {
+struct memcg_stock_pcp {
+	struct mem_cgroup *cached; /* this never be root cgroup */
+	unsigned int nr_pages;
+
 #ifdef CONFIG_MEMCG_KMEM
 	struct obj_cgroup *cached_objcg;
 	struct pglist_data *cached_pgdat;
 	unsigned int nr_bytes;
 	int nr_slab_reclaimable_b;
 	int nr_slab_unreclaimable_b;
-#else
-	int dummy[0];
 #endif
-};
-
-struct memcg_stock_pcp {
-	struct mem_cgroup *cached; /* this never be root cgroup */
-	unsigned int nr_pages;
-	struct obj_stock task_obj;
-	struct obj_stock irq_obj;
 
 	struct work_struct work;
 	unsigned long flags;
@@ -2111,12 +2105,12 @@ static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
 static DEFINE_MUTEX(percpu_charge_mutex);
 
 #ifdef CONFIG_MEMCG_KMEM
-static void drain_obj_stock(struct obj_stock *stock);
+static void drain_obj_stock(struct memcg_stock_pcp *stock);
 static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 				     struct mem_cgroup *root_memcg);
 
 #else
-static inline void drain_obj_stock(struct obj_stock *stock)
+static inline void drain_obj_stock(struct memcg_stock_pcp *stock)
 {
 }
 static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
@@ -2193,9 +2187,7 @@ static void drain_local_stock(struct work_struct *dummy)
 	local_irq_save(flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
-	drain_obj_stock(&stock->irq_obj);
-	if (in_task())
-		drain_obj_stock(&stock->task_obj);
+	drain_obj_stock(stock);
 	drain_stock(stock);
 	clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);
 
@@ -2770,41 +2762,6 @@ static struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
  */
 #define OBJCGS_CLEAR_MASK	(__GFP_DMA | __GFP_RECLAIMABLE | __GFP_ACCOUNT)
 
-/*
- * Most kmem_cache_alloc() calls are from user context. The irq disable/enable
- * sequence used in this case to access content from object stock is slow.
- * To optimize for user context access, there are now two object stocks for
- * task context and interrupt context access respectively.
- *
- * The task context object stock can be accessed by disabling preemption only
- * which is cheap in non-preempt kernel. The interrupt context object stock
- * can only be accessed after disabling interrupt. User context code can
- * access interrupt object stock, but not vice versa.
- */
-static inline struct obj_stock *get_obj_stock(unsigned long *pflags)
-{
-	struct memcg_stock_pcp *stock;
-
-	if (likely(in_task())) {
-		*pflags = 0UL;
-		preempt_disable();
-		stock = this_cpu_ptr(&memcg_stock);
-		return &stock->task_obj;
-	}
-
-	local_irq_save(*pflags);
-	stock = this_cpu_ptr(&memcg_stock);
-	return &stock->irq_obj;
-}
-
-static inline void put_obj_stock(unsigned long flags)
-{
-	if (likely(in_task()))
-		preempt_enable();
-	else
-		local_irq_restore(flags);
-}
-
 /*
  * mod_objcg_mlstate() may be called with irq enabled, so
  * mod_memcg_lruvec_state() should be used.
@@ -3075,10 +3032,13 @@ void __memcg_kmem_uncharge_page(struct page *page, int order)
 void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
 		     enum node_stat_item idx, int nr)
 {
+	struct memcg_stock_pcp *stock;
 	unsigned long flags;
-	struct obj_stock *stock = get_obj_stock(&flags);
 	int *bytes;
 
+	local_irq_save(flags);
+	stock = this_cpu_ptr(&memcg_stock);
+
 	/*
 	 * Save vmstat data in stock and skip vmstat array update unless
 	 * accumulating over a page of vmstat data or when pgdat or idx
@@ -3129,26 +3089,29 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
 	if (nr)
 		mod_objcg_mlstate(objcg, pgdat, idx, nr);
 
-	put_obj_stock(flags);
+	local_irq_restore(flags);
 }
 
 static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
 {
+	struct memcg_stock_pcp *stock;
 	unsigned long flags;
-	struct obj_stock *stock = get_obj_stock(&flags);
 	bool ret = false;
 
+	local_irq_save(flags);
+
+	stock = this_cpu_ptr(&memcg_stock);
 	if (objcg == stock->cached_objcg && stock->nr_bytes >= nr_bytes) {
 		stock->nr_bytes -= nr_bytes;
 		ret = true;
 	}
 
-	put_obj_stock(flags);
+	local_irq_restore(flags);
 
 	return ret;
 }
 
-static void drain_obj_stock(struct obj_stock *stock)
+static void drain_obj_stock(struct memcg_stock_pcp *stock)
 {
 	struct obj_cgroup *old = stock->cached_objcg;
 
@@ -3204,13 +3167,8 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 {
 	struct mem_cgroup *memcg;
 
-	if (in_task() && stock->task_obj.cached_objcg) {
-		memcg = obj_cgroup_memcg(stock->task_obj.cached_objcg);
-		if (memcg && mem_cgroup_is_descendant(memcg, root_memcg))
-			return true;
-	}
-	if (stock->irq_obj.cached_objcg) {
-		memcg = obj_cgroup_memcg(stock->irq_obj.cached_objcg);
+	if (stock->cached_objcg) {
+		memcg = obj_cgroup_memcg(stock->cached_objcg);
 		if (memcg && mem_cgroup_is_descendant(memcg, root_memcg))
 			return true;
 	}
@@ -3221,10 +3179,13 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
 			     bool allow_uncharge)
 {
+	struct memcg_stock_pcp *stock;
 	unsigned long flags;
-	struct obj_stock *stock = get_obj_stock(&flags);
 	unsigned int nr_pages = 0;
 
+	local_irq_save(flags);
+
+	stock = this_cpu_ptr(&memcg_stock);
 	if (stock->cached_objcg != objcg) { /* reset if necessary */
 		drain_obj_stock(stock);
 		obj_cgroup_get(objcg);
@@ -3240,7 +3201,7 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
 		stock->nr_bytes &= (PAGE_SIZE - 1);
 	}
 
-	put_obj_stock(flags);
+	local_irq_restore(flags);
 
 	if (nr_pages)
 		obj_cgroup_uncharge_pages(objcg, nr_pages);
@@ -6821,7 +6782,6 @@ static void uncharge_folio(struct folio *folio, struct uncharge_gather *ug)
 	long nr_pages;
 	struct mem_cgroup *memcg;
 	struct obj_cgroup *objcg;
-	bool use_objcg = folio_memcg_kmem(folio);
 
 	VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
 
@@ -6830,7 +6790,7 @@ static void uncharge_folio(struct folio *folio, struct uncharge_gather *ug)
 	 * folio memcg or objcg at this point, we have fully
 	 * exclusive access to the folio.
 	 */
-	if (use_objcg) {
+	if (folio_memcg_kmem(folio)) {
 		objcg = __folio_objcg(folio);
 		/*
 		 * This get matches the put at the end of the function and
@@ -6858,7 +6818,7 @@ static void uncharge_folio(struct folio *folio, struct uncharge_gather *ug)
 
 	nr_pages = folio_nr_pages(folio);
 
-	if (use_objcg) {
+	if (folio_memcg_kmem(folio)) {
 		ug->nr_memory += nr_pages;
 		ug->nr_kmem += nr_pages;
 
-- 
2.34.1

[PATCH v2 2/4] mm/memcg: Disable threshold event handlers on PREEMPT_RT

[Sebastian Andrzej Siewior]

During the integration of PREEMPT_RT support, the code flow around
memcg_check_events() resulted in `twisted code'. Moving the code around
and avoiding then would then lead to an additional local-irq-save
section within memcg_check_events(). While looking better, it adds a
local-irq-save section to code flow which is usually within an
local-irq-off block on non-PREEMPT_RT configurations.

The threshold event handler is a deprecated memcg v1 feature. Instead of
trying to get it to work under PREEMPT_RT just disable it. There should
be no users on PREEMPT_RT. From that perspective it makes even less
sense to get it to work under PREEMPT_RT while having zero users.

Make memory.soft_limit_in_bytes and cgroup.event_control return
-EOPNOTSUPP on PREEMPT_RT. Make an empty memcg_check_events() and
memcg_write_event_control() which return only -EOPNOTSUPP on PREEMPT_RT.
Document that the two knobs are disabled on PREEMPT_RT.

Suggested-by: Michal Hocko <mhocko@kernel.org>
Suggested-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 Documentation/admin-guide/cgroup-v1/memory.rst |  2 ++
 mm/memcontrol.c                                | 14 ++++++++++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst
index faac50149a222..2cc502a75ef64 100644
--- a/Documentation/admin-guide/cgroup-v1/memory.rst
+++ b/Documentation/admin-guide/cgroup-v1/memory.rst
@@ -64,6 +64,7 @@ Brief summary of control files.
 				     threads
  cgroup.procs			     show list of processes
  cgroup.event_control		     an interface for event_fd()
+				     This knob is not available on CONFIG_PREEMPT_RT systems.
  memory.usage_in_bytes		     show current usage for memory
 				     (See 5.5 for details)
  memory.memsw.usage_in_bytes	     show current usage for memory+Swap
@@ -75,6 +76,7 @@ Brief summary of control files.
  memory.max_usage_in_bytes	     show max memory usage recorded
  memory.memsw.max_usage_in_bytes     show max memory+Swap usage recorded
  memory.soft_limit_in_bytes	     set/show soft limit of memory usage
+				     This knob is not available on CONFIG_PREEMPT_RT systems.
  memory.stat			     show various statistics
  memory.use_hierarchy		     set/show hierarchical account enabled
                                      This knob is deprecated and shouldn't be
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4b1572ae990d8..c1caa662946dc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -858,6 +858,9 @@ static bool mem_cgroup_event_ratelimit(struct mem_cgroup *memcg,
  */
 static void memcg_check_events(struct mem_cgroup *memcg, int nid)
 {
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		return;
+
 	/* threshold event is triggered in finer grain than soft limit */
 	if (unlikely(mem_cgroup_event_ratelimit(memcg,
 						MEM_CGROUP_TARGET_THRESH))) {
@@ -3724,8 +3727,12 @@ static ssize_t mem_cgroup_write(struct kernfs_open_file *of,
 		}
 		break;
 	case RES_SOFT_LIMIT:
-		memcg->soft_limit = nr_pages;
-		ret = 0;
+		if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
+			ret = -EOPNOTSUPP;
+		} else {
+			memcg->soft_limit = nr_pages;
+			ret = 0;
+		}
 		break;
 	}
 	return ret ?: nbytes;
@@ -4701,6 +4708,9 @@ static ssize_t memcg_write_event_control(struct kernfs_open_file *of,
 	char *endp;
 	int ret;
 
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		return -EOPNOTSUPP;
+
 	buf = strstrip(buf);
 
 	efd = simple_strtoul(buf, &endp, 10);
-- 
2.34.1

[PATCH v2 2/4] mm/memcg: Disable threshold event handlers on PREEMPT_RT

[Sebastian Andrzej Siewior]

During the integration of PREEMPT_RT support, the code flow around
memcg_check_events() resulted in `twisted code'. Moving the code around
and avoiding then would then lead to an additional local-irq-save
section within memcg_check_events(). While looking better, it adds a
local-irq-save section to code flow which is usually within an
local-irq-off block on non-PREEMPT_RT configurations.

The threshold event handler is a deprecated memcg v1 feature. Instead of
trying to get it to work under PREEMPT_RT just disable it. There should
be no users on PREEMPT_RT. From that perspective it makes even less
sense to get it to work under PREEMPT_RT while having zero users.

Make memory.soft_limit_in_bytes and cgroup.event_control return
-EOPNOTSUPP on PREEMPT_RT. Make an empty memcg_check_events() and
memcg_write_event_control() which return only -EOPNOTSUPP on PREEMPT_RT.
Document that the two knobs are disabled on PREEMPT_RT.

Suggested-by: Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Suggested-by: Michal Koutn√Ω <mkoutny-IBi9RG/b67k@public.gmane.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
---
 Documentation/admin-guide/cgroup-v1/memory.rst |  2 ++
 mm/memcontrol.c                                | 14 ++++++++++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst
index faac50149a222..2cc502a75ef64 100644
--- a/Documentation/admin-guide/cgroup-v1/memory.rst
+++ b/Documentation/admin-guide/cgroup-v1/memory.rst
@@ -64,6 +64,7 @@ Brief summary of control files.
 				     threads
  cgroup.procs			     show list of processes
  cgroup.event_control		     an interface for event_fd()
+				     This knob is not available on CONFIG_PREEMPT_RT systems.
  memory.usage_in_bytes		     show current usage for memory
 				     (See 5.5 for details)
  memory.memsw.usage_in_bytes	     show current usage for memory+Swap
@@ -75,6 +76,7 @@ Brief summary of control files.
  memory.max_usage_in_bytes	     show max memory usage recorded
  memory.memsw.max_usage_in_bytes     show max memory+Swap usage recorded
  memory.soft_limit_in_bytes	     set/show soft limit of memory usage
+				     This knob is not available on CONFIG_PREEMPT_RT systems.
  memory.stat			     show various statistics
  memory.use_hierarchy		     set/show hierarchical account enabled
                                      This knob is deprecated and shouldn't be
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4b1572ae990d8..c1caa662946dc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -858,6 +858,9 @@ static bool mem_cgroup_event_ratelimit(struct mem_cgroup *memcg,
  */
 static void memcg_check_events(struct mem_cgroup *memcg, int nid)
 {
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		return;
+
 	/* threshold event is triggered in finer grain than soft limit */
 	if (unlikely(mem_cgroup_event_ratelimit(memcg,
 						MEM_CGROUP_TARGET_THRESH))) {
@@ -3724,8 +3727,12 @@ static ssize_t mem_cgroup_write(struct kernfs_open_file *of,
 		}
 		break;
 	case RES_SOFT_LIMIT:
-		memcg->soft_limit = nr_pages;
-		ret = 0;
+		if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
+			ret = -EOPNOTSUPP;
+		} else {
+			memcg->soft_limit = nr_pages;
+			ret = 0;
+		}
 		break;
 	}
 	return ret ?: nbytes;
@@ -4701,6 +4708,9 @@ static ssize_t memcg_write_event_control(struct kernfs_open_file *of,
 	char *endp;
 	int ret;
 
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		return -EOPNOTSUPP;
+
 	buf = strstrip(buf);
 
 	efd = simple_strtoul(buf, &endp, 10);
-- 
2.34.1

[PATCH v2 3/4] mm/memcg: Protect per-CPU counter by disabling preemption on PREEMPT_RT where needed.

[Sebastian Andrzej Siewior]

The per-CPU counter are modified with the non-atomic modifier. The
consistency is ensured by disabling interrupts for the update.
On non PREEMPT_RT configuration this works because acquiring a
spinlock_t typed lock with the _irq() suffix disables interrupts. On
PREEMPT_RT configurations the RMW operation can be interrupted.

Another problem is that mem_cgroup_swapout() expects to be invoked with
disabled interrupts because the caller has to acquire a spinlock_t which
is acquired with disabled interrupts. Since spinlock_t never disables
interrupts on PREEMPT_RT the interrupts are never disabled at this
point.

The code is never called from in_irq() context on PREEMPT_RT therefore
disabling preemption during the update is sufficient on PREEMPT_RT.
The sections which explicitly disable interrupts can remain on
PREEMPT_RT because the sections remain short and they don't involve
sleeping locks (memcg_check_events() is doing nothing on PREEMPT_RT).

Disable preemption during update of the per-CPU variables which do not
explicitly disable interrupts.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 mm/memcontrol.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c1caa662946dc..466466f285cea 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -705,6 +705,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
 	memcg = pn->memcg;
 
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_disable();
 	/* Update memcg */
 	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
 
@@ -712,6 +714,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
 
 	memcg_rstat_updated(memcg, val);
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable();
 }
 
 /**
@@ -794,8 +798,12 @@ void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
 	if (mem_cgroup_disabled())
 		return;
 
+	if (IS_ENABLED(PREEMPT_RT))
+		preempt_disable();
 	__this_cpu_add(memcg->vmstats_percpu->events[idx], count);
 	memcg_rstat_updated(memcg, count);
+	if (IS_ENABLED(PREEMPT_RT))
+		preempt_enable();
 }
 
 static unsigned long memcg_events(struct mem_cgroup *memcg, int event)
@@ -7148,9 +7156,18 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
 	 * i_pages lock which is taken with interrupts-off. It is
 	 * important here to have the interrupts disabled because it is the
 	 * only synchronisation we have for updating the per-CPU variables.
+	 * On PREEMPT_RT interrupts are never disabled and the updates to per-CPU
+	 * variables are synchronised by keeping preemption disabled.
 	 */
-	VM_BUG_ON(!irqs_disabled());
-	mem_cgroup_charge_statistics(memcg, -nr_entries);
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
+		VM_BUG_ON(!irqs_disabled());
+		mem_cgroup_charge_statistics(memcg, -nr_entries);
+	} else {
+		preempt_disable();
+		mem_cgroup_charge_statistics(memcg, -nr_entries);
+		preempt_enable();
+	}
+
 	memcg_check_events(memcg, page_to_nid(page));
 
 	css_put(&memcg->css);
-- 
2.34.1

[PATCH v2 3/4] mm/memcg: Protect per-CPU counter by disabling preemption on PREEMPT_RT where needed.

[Sebastian Andrzej Siewior]

The per-CPU counter are modified with the non-atomic modifier. The
consistency is ensured by disabling interrupts for the update.
On non PREEMPT_RT configuration this works because acquiring a
spinlock_t typed lock with the _irq() suffix disables interrupts. On
PREEMPT_RT configurations the RMW operation can be interrupted.

Another problem is that mem_cgroup_swapout() expects to be invoked with
disabled interrupts because the caller has to acquire a spinlock_t which
is acquired with disabled interrupts. Since spinlock_t never disables
interrupts on PREEMPT_RT the interrupts are never disabled at this
point.

The code is never called from in_irq() context on PREEMPT_RT therefore
disabling preemption during the update is sufficient on PREEMPT_RT.
The sections which explicitly disable interrupts can remain on
PREEMPT_RT because the sections remain short and they don't involve
sleeping locks (memcg_check_events() is doing nothing on PREEMPT_RT).

Disable preemption during update of the per-CPU variables which do not
explicitly disable interrupts.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
---
 mm/memcontrol.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c1caa662946dc..466466f285cea 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -705,6 +705,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
 	memcg = pn->memcg;
 
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_disable();
 	/* Update memcg */
 	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
 
@@ -712,6 +714,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
 
 	memcg_rstat_updated(memcg, val);
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable();
 }
 
 /**
@@ -794,8 +798,12 @@ void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
 	if (mem_cgroup_disabled())
 		return;
 
+	if (IS_ENABLED(PREEMPT_RT))
+		preempt_disable();
 	__this_cpu_add(memcg->vmstats_percpu->events[idx], count);
 	memcg_rstat_updated(memcg, count);
+	if (IS_ENABLED(PREEMPT_RT))
+		preempt_enable();
 }
 
 static unsigned long memcg_events(struct mem_cgroup *memcg, int event)
@@ -7148,9 +7156,18 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
 	 * i_pages lock which is taken with interrupts-off. It is
 	 * important here to have the interrupts disabled because it is the
 	 * only synchronisation we have for updating the per-CPU variables.
+	 * On PREEMPT_RT interrupts are never disabled and the updates to per-CPU
+	 * variables are synchronised by keeping preemption disabled.
 	 */
-	VM_BUG_ON(!irqs_disabled());
-	mem_cgroup_charge_statistics(memcg, -nr_entries);
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
+		VM_BUG_ON(!irqs_disabled());
+		mem_cgroup_charge_statistics(memcg, -nr_entries);
+	} else {
+		preempt_disable();
+		mem_cgroup_charge_statistics(memcg, -nr_entries);
+		preempt_enable();
+	}
+
 	memcg_check_events(memcg, page_to_nid(page));
 
 	css_put(&memcg->css);
-- 
2.34.1

[PATCH v2 4/4] mm/memcg: Protect memcg_stock with a local_lock_t

[Sebastian Andrzej Siewior]

The members of the per-CPU structure memcg_stock_pcp are protected by
disabling interrupts. This is not working on PREEMPT_RT because it
creates atomic context in which actions are performed which require
preemptible context. One example is obj_cgroup_release().

The IRQ-disable sections can be replaced with local_lock_t which
preserves the explicit disabling of interrupts while keeps the code
preemptible on PREEMPT_RT.

drain_all_stock() disables preemption via get_cpu() and then invokes
drain_local_stock() if it is the local CPU to avoid scheduling a worker (which
invokes the same function). Disabling preemption here is problematic due to the
sleeping locks in drain_local_stock().
This can be avoided by always scheduling a worker, even for the local
CPU. Using cpus_read_lock() to stabilize the cpu_online_mask is not
needed since the worker operates always on the CPU-local data structure.
Should a CPU go offline then a two worker would perform the work and no
harm is done. Using cpus_read_lock() leads to a possible deadlock.

drain_obj_stock() drops a reference on obj_cgroup which leads to an invocation
of obj_cgroup_release() if it is the last object. This in turn leads to
recursive locking of the local_lock_t. To avoid this, obj_cgroup_release() is
invoked outside of the locked section.

obj_cgroup_uncharge_pages() can be invoked with the local_lock_t acquired and
without it. This will lead later to a recursion in refill_stock(). To
avoid the locking recursion provide obj_cgroup_uncharge_pages_locked()
which uses the locked version of refill_stock().

- Replace disabling interrupts for memcg_stock with a local_lock_t.

- Schedule a worker even for the local CPU instead of invoking it
  directly (in drain_all_stock()).

- Let drain_obj_stock() return the old struct obj_cgroup which is passed
  to obj_cgroup_put() outside of the locked section.

- Provide obj_cgroup_uncharge_pages_locked() which uses the locked
  version of refill_stock() to avoid recursive locking in
  drain_obj_stock().

Link: https://lkml.kernel.org/r/20220209014709.GA26885@xsang-OptiPlex-9020
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 mm/memcontrol.c | 101 ++++++++++++++++++++++++++++++------------------
 1 file changed, 63 insertions(+), 38 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 466466f285cea..f7120a92cf46e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2097,6 +2097,7 @@ void unlock_page_memcg(struct page *page)
 }
 
 struct memcg_stock_pcp {
+	local_lock_t stock_lock;
 	struct mem_cgroup *cached; /* this never be root cgroup */
 	unsigned int nr_pages;
 
@@ -2112,17 +2113,20 @@ struct memcg_stock_pcp {
 	unsigned long flags;
 #define FLUSHING_CACHED_CHARGE	0
 };
-static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
+static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock) = {
+	.stock_lock = INIT_LOCAL_LOCK(stock_lock),
+};
 static DEFINE_MUTEX(percpu_charge_mutex);
 
 #ifdef CONFIG_MEMCG_KMEM
-static void drain_obj_stock(struct memcg_stock_pcp *stock);
+static struct obj_cgroup *drain_obj_stock(struct memcg_stock_pcp *stock);
 static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 				     struct mem_cgroup *root_memcg);
 
 #else
-static inline void drain_obj_stock(struct memcg_stock_pcp *stock)
+static inline struct obj_cgroup *drain_obj_stock(struct memcg_stock_pcp *stock)
 {
+	return NULL;
 }
 static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 				     struct mem_cgroup *root_memcg)
@@ -2151,7 +2155,7 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 	if (nr_pages > MEMCG_CHARGE_BATCH)
 		return ret;
 
-	local_irq_save(flags);
+	local_lock_irqsave(&memcg_stock.stock_lock, flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
 	if (memcg == stock->cached && stock->nr_pages >= nr_pages) {
@@ -2159,7 +2163,7 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 		ret = true;
 	}
 
-	local_irq_restore(flags);
+	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
 
 	return ret;
 }
@@ -2188,6 +2192,7 @@ static void drain_stock(struct memcg_stock_pcp *stock)
 static void drain_local_stock(struct work_struct *dummy)
 {
 	struct memcg_stock_pcp *stock;
+	struct obj_cgroup *old = NULL;
 	unsigned long flags;
 
 	/*
@@ -2195,26 +2200,25 @@ static void drain_local_stock(struct work_struct *dummy)
 	 * drain_stock races is that we always operate on local CPU stock
 	 * here with IRQ disabled
 	 */
-	local_irq_save(flags);
+	local_lock_irqsave(&memcg_stock.stock_lock, flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
-	drain_obj_stock(stock);
+	old = drain_obj_stock(stock);
 	drain_stock(stock);
 	clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);
 
-	local_irq_restore(flags);
+	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
+	if (old)
+		obj_cgroup_put(old);
 }
 
 /*
  * Cache charges(val) to local per_cpu area.
  * This will be consumed by consume_stock() function, later.
  */
-static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
+static void __refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 {
 	struct memcg_stock_pcp *stock;
-	unsigned long flags;
-
-	local_irq_save(flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
 	if (stock->cached != memcg) { /* reset if necessary */
@@ -2226,8 +2230,15 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 
 	if (stock->nr_pages > MEMCG_CHARGE_BATCH)
 		drain_stock(stock);
+}
 
-	local_irq_restore(flags);
+static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
+{
+	unsigned long flags;
+
+	local_lock_irqsave(&memcg_stock.stock_lock, flags);
+	__refill_stock(memcg, nr_pages);
+	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
 }
 
 /*
@@ -2236,7 +2247,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
  */
 static void drain_all_stock(struct mem_cgroup *root_memcg)
 {
-	int cpu, curcpu;
+	int cpu;
 
 	/* If someone's already draining, avoid adding running more workers. */
 	if (!mutex_trylock(&percpu_charge_mutex))
@@ -2247,7 +2258,6 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
 	 * as well as workers from this path always operate on the local
 	 * per-cpu data. CPU up doesn't touch memcg_stock at all.
 	 */
-	curcpu = get_cpu();
 	for_each_online_cpu(cpu) {
 		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
 		struct mem_cgroup *memcg;
@@ -2263,14 +2273,9 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
 		rcu_read_unlock();
 
 		if (flush &&
-		    !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
-			if (cpu == curcpu)
-				drain_local_stock(&stock->work);
-			else
-				schedule_work_on(cpu, &stock->work);
-		}
+		    !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
+			schedule_work_on(cpu, &stock->work);
 	}
-	put_cpu();
 	mutex_unlock(&percpu_charge_mutex);
 }
 
@@ -2948,12 +2953,13 @@ static void memcg_free_cache_id(int id)
 }
 
 /*
- * obj_cgroup_uncharge_pages: uncharge a number of kernel pages from a objcg
+ * __obj_cgroup_uncharge_pages: uncharge a number of kernel pages from a objcg
  * @objcg: object cgroup to uncharge
  * @nr_pages: number of pages to uncharge
  */
-static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
-				      unsigned int nr_pages)
+static void __obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
+					unsigned int nr_pages,
+					void (*refill)(struct mem_cgroup *memcg, unsigned int nr_pages))
 {
 	struct mem_cgroup *memcg;
 
@@ -2961,11 +2967,24 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
 
 	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
 		page_counter_uncharge(&memcg->kmem, nr_pages);
-	refill_stock(memcg, nr_pages);
+	refill(memcg, nr_pages);
 
 	css_put(&memcg->css);
 }
 
+static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
+				      unsigned int nr_pages)
+{
+	__obj_cgroup_uncharge_pages(objcg, nr_pages, refill_stock);
+}
+
+static void obj_cgroup_uncharge_pages_locked(struct obj_cgroup *objcg,
+					     unsigned int nr_pages)
+{
+	__obj_cgroup_uncharge_pages(objcg, nr_pages, __refill_stock);
+}
+
+
 /*
  * obj_cgroup_charge_pages: charge a number of kernel pages to a objcg
  * @objcg: object cgroup to charge
@@ -3044,10 +3063,11 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
 		     enum node_stat_item idx, int nr)
 {
 	struct memcg_stock_pcp *stock;
+	struct obj_cgroup *old = NULL;
 	unsigned long flags;
 	int *bytes;
 
-	local_irq_save(flags);
+	local_lock_irqsave(&memcg_stock.stock_lock, flags);
 	stock = this_cpu_ptr(&memcg_stock);
 
 	/*
@@ -3056,7 +3076,7 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
 	 * changes.
 	 */
 	if (stock->cached_objcg != objcg) {
-		drain_obj_stock(stock);
+		old = drain_obj_stock(stock);
 		obj_cgroup_get(objcg);
 		stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes)
 				? atomic_xchg(&objcg->nr_charged_bytes, 0) : 0;
@@ -3100,7 +3120,9 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
 	if (nr)
 		mod_objcg_mlstate(objcg, pgdat, idx, nr);
 
-	local_irq_restore(flags);
+	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
+	if (old)
+		obj_cgroup_put(old);
 }
 
 static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
@@ -3109,7 +3131,7 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
 	unsigned long flags;
 	bool ret = false;
 
-	local_irq_save(flags);
+	local_lock_irqsave(&memcg_stock.stock_lock, flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
 	if (objcg == stock->cached_objcg && stock->nr_bytes >= nr_bytes) {
@@ -3117,24 +3139,24 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
 		ret = true;
 	}
 
-	local_irq_restore(flags);
+	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
 
 	return ret;
 }
 
-static void drain_obj_stock(struct memcg_stock_pcp *stock)
+static struct obj_cgroup *drain_obj_stock(struct memcg_stock_pcp *stock)
 {
 	struct obj_cgroup *old = stock->cached_objcg;
 
 	if (!old)
-		return;
+		return NULL;
 
 	if (stock->nr_bytes) {
 		unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
 		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
 
 		if (nr_pages)
-			obj_cgroup_uncharge_pages(old, nr_pages);
+			obj_cgroup_uncharge_pages_locked(old, nr_pages);
 
 		/*
 		 * The leftover is flushed to the centralized per-memcg value.
@@ -3169,8 +3191,8 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
 		stock->cached_pgdat = NULL;
 	}
 
-	obj_cgroup_put(old);
 	stock->cached_objcg = NULL;
+	return old;
 }
 
 static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
@@ -3191,14 +3213,15 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
 			     bool allow_uncharge)
 {
 	struct memcg_stock_pcp *stock;
+	struct obj_cgroup *old = NULL;
 	unsigned long flags;
 	unsigned int nr_pages = 0;
 
-	local_irq_save(flags);
+	local_lock_irqsave(&memcg_stock.stock_lock, flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
 	if (stock->cached_objcg != objcg) { /* reset if necessary */
-		drain_obj_stock(stock);
+		old = drain_obj_stock(stock);
 		obj_cgroup_get(objcg);
 		stock->cached_objcg = objcg;
 		stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes)
@@ -3212,7 +3235,9 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
 		stock->nr_bytes &= (PAGE_SIZE - 1);
 	}
 
-	local_irq_restore(flags);
+	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
+	if (old)
+		obj_cgroup_put(old);
 
 	if (nr_pages)
 		obj_cgroup_uncharge_pages(objcg, nr_pages);
-- 
2.34.1

[PATCH v2 4/4] mm/memcg: Protect memcg_stock with a local_lock_t

[Sebastian Andrzej Siewior]

The members of the per-CPU structure memcg_stock_pcp are protected by
disabling interrupts. This is not working on PREEMPT_RT because it
creates atomic context in which actions are performed which require
preemptible context. One example is obj_cgroup_release().

The IRQ-disable sections can be replaced with local_lock_t which
preserves the explicit disabling of interrupts while keeps the code
preemptible on PREEMPT_RT.

drain_all_stock() disables preemption via get_cpu() and then invokes
drain_local_stock() if it is the local CPU to avoid scheduling a worker (which
invokes the same function). Disabling preemption here is problematic due to the
sleeping locks in drain_local_stock().
This can be avoided by always scheduling a worker, even for the local
CPU. Using cpus_read_lock() to stabilize the cpu_online_mask is not
needed since the worker operates always on the CPU-local data structure.
Should a CPU go offline then a two worker would perform the work and no
harm is done. Using cpus_read_lock() leads to a possible deadlock.

drain_obj_stock() drops a reference on obj_cgroup which leads to an invocation
of obj_cgroup_release() if it is the last object. This in turn leads to
recursive locking of the local_lock_t. To avoid this, obj_cgroup_release() is
invoked outside of the locked section.

obj_cgroup_uncharge_pages() can be invoked with the local_lock_t acquired and
without it. This will lead later to a recursion in refill_stock(). To
avoid the locking recursion provide obj_cgroup_uncharge_pages_locked()
which uses the locked version of refill_stock().

- Replace disabling interrupts for memcg_stock with a local_lock_t.

- Schedule a worker even for the local CPU instead of invoking it
  directly (in drain_all_stock()).

- Let drain_obj_stock() return the old struct obj_cgroup which is passed
  to obj_cgroup_put() outside of the locked section.

- Provide obj_cgroup_uncharge_pages_locked() which uses the locked
  version of refill_stock() to avoid recursive locking in
  drain_obj_stock().

Link: https://lkml.kernel.org/r/20220209014709.GA26885@xsang-OptiPlex-9020
Reported-by: kernel test robot <oliver.sang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
---
 mm/memcontrol.c | 101 ++++++++++++++++++++++++++++++------------------
 1 file changed, 63 insertions(+), 38 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 466466f285cea..f7120a92cf46e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2097,6 +2097,7 @@ void unlock_page_memcg(struct page *page)
 }
 
 struct memcg_stock_pcp {
+	local_lock_t stock_lock;
 	struct mem_cgroup *cached; /* this never be root cgroup */
 	unsigned int nr_pages;
 
@@ -2112,17 +2113,20 @@ struct memcg_stock_pcp {
 	unsigned long flags;
 #define FLUSHING_CACHED_CHARGE	0
 };
-static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
+static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock) = {
+	.stock_lock = INIT_LOCAL_LOCK(stock_lock),
+};
 static DEFINE_MUTEX(percpu_charge_mutex);
 
 #ifdef CONFIG_MEMCG_KMEM
-static void drain_obj_stock(struct memcg_stock_pcp *stock);
+static struct obj_cgroup *drain_obj_stock(struct memcg_stock_pcp *stock);
 static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 				     struct mem_cgroup *root_memcg);
 
 #else
-static inline void drain_obj_stock(struct memcg_stock_pcp *stock)
+static inline struct obj_cgroup *drain_obj_stock(struct memcg_stock_pcp *stock)
 {
+	return NULL;
 }
 static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 				     struct mem_cgroup *root_memcg)
@@ -2151,7 +2155,7 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 	if (nr_pages > MEMCG_CHARGE_BATCH)
 		return ret;
 
-	local_irq_save(flags);
+	local_lock_irqsave(&memcg_stock.stock_lock, flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
 	if (memcg == stock->cached && stock->nr_pages >= nr_pages) {
@@ -2159,7 +2163,7 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 		ret = true;
 	}
 
-	local_irq_restore(flags);
+	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
 
 	return ret;
 }
@@ -2188,6 +2192,7 @@ static void drain_stock(struct memcg_stock_pcp *stock)
 static void drain_local_stock(struct work_struct *dummy)
 {
 	struct memcg_stock_pcp *stock;
+	struct obj_cgroup *old = NULL;
 	unsigned long flags;
 
 	/*
@@ -2195,26 +2200,25 @@ static void drain_local_stock(struct work_struct *dummy)
 	 * drain_stock races is that we always operate on local CPU stock
 	 * here with IRQ disabled
 	 */
-	local_irq_save(flags);
+	local_lock_irqsave(&memcg_stock.stock_lock, flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
-	drain_obj_stock(stock);
+	old = drain_obj_stock(stock);
 	drain_stock(stock);
 	clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);
 
-	local_irq_restore(flags);
+	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
+	if (old)
+		obj_cgroup_put(old);
 }
 
 /*
  * Cache charges(val) to local per_cpu area.
  * This will be consumed by consume_stock() function, later.
  */
-static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
+static void __refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 {
 	struct memcg_stock_pcp *stock;
-	unsigned long flags;
-
-	local_irq_save(flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
 	if (stock->cached != memcg) { /* reset if necessary */
@@ -2226,8 +2230,15 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 
 	if (stock->nr_pages > MEMCG_CHARGE_BATCH)
 		drain_stock(stock);
+}
 
-	local_irq_restore(flags);
+static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
+{
+	unsigned long flags;
+
+	local_lock_irqsave(&memcg_stock.stock_lock, flags);
+	__refill_stock(memcg, nr_pages);
+	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
 }
 
 /*
@@ -2236,7 +2247,7 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
  */
 static void drain_all_stock(struct mem_cgroup *root_memcg)
 {
-	int cpu, curcpu;
+	int cpu;
 
 	/* If someone's already draining, avoid adding running more workers. */
 	if (!mutex_trylock(&percpu_charge_mutex))
@@ -2247,7 +2258,6 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
 	 * as well as workers from this path always operate on the local
 	 * per-cpu data. CPU up doesn't touch memcg_stock at all.
 	 */
-	curcpu = get_cpu();
 	for_each_online_cpu(cpu) {
 		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
 		struct mem_cgroup *memcg;
@@ -2263,14 +2273,9 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
 		rcu_read_unlock();
 
 		if (flush &&
-		    !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
-			if (cpu == curcpu)
-				drain_local_stock(&stock->work);
-			else
-				schedule_work_on(cpu, &stock->work);
-		}
+		    !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
+			schedule_work_on(cpu, &stock->work);
 	}
-	put_cpu();
 	mutex_unlock(&percpu_charge_mutex);
 }
 
@@ -2948,12 +2953,13 @@ static void memcg_free_cache_id(int id)
 }
 
 /*
- * obj_cgroup_uncharge_pages: uncharge a number of kernel pages from a objcg
+ * __obj_cgroup_uncharge_pages: uncharge a number of kernel pages from a objcg
  * @objcg: object cgroup to uncharge
  * @nr_pages: number of pages to uncharge
  */
-static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
-				      unsigned int nr_pages)
+static void __obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
+					unsigned int nr_pages,
+					void (*refill)(struct mem_cgroup *memcg, unsigned int nr_pages))
 {
 	struct mem_cgroup *memcg;
 
@@ -2961,11 +2967,24 @@ static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
 
 	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
 		page_counter_uncharge(&memcg->kmem, nr_pages);
-	refill_stock(memcg, nr_pages);
+	refill(memcg, nr_pages);
 
 	css_put(&memcg->css);
 }
 
+static void obj_cgroup_uncharge_pages(struct obj_cgroup *objcg,
+				      unsigned int nr_pages)
+{
+	__obj_cgroup_uncharge_pages(objcg, nr_pages, refill_stock);
+}
+
+static void obj_cgroup_uncharge_pages_locked(struct obj_cgroup *objcg,
+					     unsigned int nr_pages)
+{
+	__obj_cgroup_uncharge_pages(objcg, nr_pages, __refill_stock);
+}
+
+
 /*
  * obj_cgroup_charge_pages: charge a number of kernel pages to a objcg
  * @objcg: object cgroup to charge
@@ -3044,10 +3063,11 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
 		     enum node_stat_item idx, int nr)
 {
 	struct memcg_stock_pcp *stock;
+	struct obj_cgroup *old = NULL;
 	unsigned long flags;
 	int *bytes;
 
-	local_irq_save(flags);
+	local_lock_irqsave(&memcg_stock.stock_lock, flags);
 	stock = this_cpu_ptr(&memcg_stock);
 
 	/*
@@ -3056,7 +3076,7 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
 	 * changes.
 	 */
 	if (stock->cached_objcg != objcg) {
-		drain_obj_stock(stock);
+		old = drain_obj_stock(stock);
 		obj_cgroup_get(objcg);
 		stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes)
 				? atomic_xchg(&objcg->nr_charged_bytes, 0) : 0;
@@ -3100,7 +3120,9 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
 	if (nr)
 		mod_objcg_mlstate(objcg, pgdat, idx, nr);
 
-	local_irq_restore(flags);
+	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
+	if (old)
+		obj_cgroup_put(old);
 }
 
 static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
@@ -3109,7 +3131,7 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
 	unsigned long flags;
 	bool ret = false;
 
-	local_irq_save(flags);
+	local_lock_irqsave(&memcg_stock.stock_lock, flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
 	if (objcg == stock->cached_objcg && stock->nr_bytes >= nr_bytes) {
@@ -3117,24 +3139,24 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
 		ret = true;
 	}
 
-	local_irq_restore(flags);
+	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
 
 	return ret;
 }
 
-static void drain_obj_stock(struct memcg_stock_pcp *stock)
+static struct obj_cgroup *drain_obj_stock(struct memcg_stock_pcp *stock)
 {
 	struct obj_cgroup *old = stock->cached_objcg;
 
 	if (!old)
-		return;
+		return NULL;
 
 	if (stock->nr_bytes) {
 		unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
 		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
 
 		if (nr_pages)
-			obj_cgroup_uncharge_pages(old, nr_pages);
+			obj_cgroup_uncharge_pages_locked(old, nr_pages);
 
 		/*
 		 * The leftover is flushed to the centralized per-memcg value.
@@ -3169,8 +3191,8 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
 		stock->cached_pgdat = NULL;
 	}
 
-	obj_cgroup_put(old);
 	stock->cached_objcg = NULL;
+	return old;
 }
 
 static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
@@ -3191,14 +3213,15 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
 			     bool allow_uncharge)
 {
 	struct memcg_stock_pcp *stock;
+	struct obj_cgroup *old = NULL;
 	unsigned long flags;
 	unsigned int nr_pages = 0;
 
-	local_irq_save(flags);
+	local_lock_irqsave(&memcg_stock.stock_lock, flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
 	if (stock->cached_objcg != objcg) { /* reset if necessary */
-		drain_obj_stock(stock);
+		old = drain_obj_stock(stock);
 		obj_cgroup_get(objcg);
 		stock->cached_objcg = objcg;
 		stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes)
@@ -3212,7 +3235,9 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
 		stock->nr_bytes &= (PAGE_SIZE - 1);
 	}
 
-	local_irq_restore(flags);
+	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
+	if (old)
+		obj_cgroup_put(old);
 
 	if (nr_pages)
 		obj_cgroup_uncharge_pages(objcg, nr_pages);
-- 
2.34.1

Re: [PATCH v2 4/4] mm/memcg: Protect memcg_stock with a local_lock_t

[Johannes Weiner]

Hi Sebastian,

On Fri, Feb 11, 2022 at 11:35:37PM +0100, Sebastian Andrzej Siewior wrote:
> +static struct obj_cgroup *drain_obj_stock(struct memcg_stock_pcp *stock)
>  {
>  	struct obj_cgroup *old = stock->cached_objcg;
>  
>  	if (!old)
> -		return;
> +		return NULL;
>  
>  	if (stock->nr_bytes) {
>  		unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
>  		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
>  
>  		if (nr_pages)
> -			obj_cgroup_uncharge_pages(old, nr_pages);
> +			obj_cgroup_uncharge_pages_locked(old, nr_pages);

This is a bit dubious in terms of layering. It's an objcg operation,
but what's "locked" isn't the objcg, it's the underlying stock. That
function then looks it up again, even though we have it right there.

You can open-code it and factor out the stock operation instead, and
it makes things much simpler and clearer.

I.e. something like this (untested!):

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1b3550f09335..eed6e0ff84d7 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2215,6 +2215,20 @@ static void drain_local_stock(struct work_struct *dummy)
 	local_irq_restore(flags);
 }
 
+static void __refill_stock(struct memcg_stock_pcp *stock,
+			   struct mem_cgroup *memcg,
+			   unsigned int nr_pages)
+{
+	if (stock->cached != memcg) {
+		drain_stock(stock);
+		css_get(&memcg->css);
+		stock->cached = memcg;
+	}
+	stock->nr_pages += nr_pages;
+	if (stock->nr_pages > MEMCG_CHARGE_BATCH)
+		drain_stock(stock);
+}
+
 /*
  * Cache charges(val) to local per_cpu area.
  * This will be consumed by consume_stock() function, later.
@@ -2225,18 +2239,8 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 	unsigned long flags;
 
 	local_irq_save(flags);
-
 	stock = this_cpu_ptr(&memcg_stock);
-	if (stock->cached != memcg) { /* reset if necessary */
-		drain_stock(stock);
-		css_get(&memcg->css);
-		stock->cached = memcg;
-	}
-	stock->nr_pages += nr_pages;
-
-	if (stock->nr_pages > MEMCG_CHARGE_BATCH)
-		drain_stock(stock);
-
+	__refill_stock(stock, memcg, nr_pages);
 	local_irq_restore(flags);
 }
 
@@ -3213,8 +3217,15 @@ static void drain_obj_stock(struct obj_stock *stock)
 		unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
 		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
 
-		if (nr_pages)
-			obj_cgroup_uncharge_pages(old, nr_pages);
+		/* Flush complete pages back to the page stock */
+		if (nr_pages) {
+			struct mem_cgroup *memcg;
+
+			memcg = get_mem_cgroup_from_objcg(objcg);
+			mem_cgroup_kmem_record(memcg, -nr_pages);
+			__refill_stock(stock, memcg, nr_pages);
+			css_put(&memcg->css);
+		}
 
 		/*
 		 * The leftover is flushed to the centralized per-memcg value.

Re: [PATCH v2 4/4] mm/memcg: Protect memcg_stock with a local_lock_t

[Johannes Weiner]

Hi Sebastian,

On Fri, Feb 11, 2022 at 11:35:37PM +0100, Sebastian Andrzej Siewior wrote:
> +static struct obj_cgroup *drain_obj_stock(struct memcg_stock_pcp *stock)
>  {
>  	struct obj_cgroup *old = stock->cached_objcg;
>  
>  	if (!old)
> -		return;
> +		return NULL;
>  
>  	if (stock->nr_bytes) {
>  		unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
>  		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
>  
>  		if (nr_pages)
> -			obj_cgroup_uncharge_pages(old, nr_pages);
> +			obj_cgroup_uncharge_pages_locked(old, nr_pages);

This is a bit dubious in terms of layering. It's an objcg operation,
but what's "locked" isn't the objcg, it's the underlying stock. That
function then looks it up again, even though we have it right there.

You can open-code it and factor out the stock operation instead, and
it makes things much simpler and clearer.

I.e. something like this (untested!):

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1b3550f09335..eed6e0ff84d7 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2215,6 +2215,20 @@ static void drain_local_stock(struct work_struct *dummy)
 	local_irq_restore(flags);
 }
 
+static void __refill_stock(struct memcg_stock_pcp *stock,
+			   struct mem_cgroup *memcg,
+			   unsigned int nr_pages)
+{
+	if (stock->cached != memcg) {
+		drain_stock(stock);
+		css_get(&memcg->css);
+		stock->cached = memcg;
+	}
+	stock->nr_pages += nr_pages;
+	if (stock->nr_pages > MEMCG_CHARGE_BATCH)
+		drain_stock(stock);
+}
+
 /*
  * Cache charges(val) to local per_cpu area.
  * This will be consumed by consume_stock() function, later.
@@ -2225,18 +2239,8 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 	unsigned long flags;
 
 	local_irq_save(flags);
-
 	stock = this_cpu_ptr(&memcg_stock);
-	if (stock->cached != memcg) { /* reset if necessary */
-		drain_stock(stock);
-		css_get(&memcg->css);
-		stock->cached = memcg;
-	}
-	stock->nr_pages += nr_pages;
-
-	if (stock->nr_pages > MEMCG_CHARGE_BATCH)
-		drain_stock(stock);
-
+	__refill_stock(stock, memcg, nr_pages);
 	local_irq_restore(flags);
 }
 
@@ -3213,8 +3217,15 @@ static void drain_obj_stock(struct obj_stock *stock)
 		unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
 		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
 
-		if (nr_pages)
-			obj_cgroup_uncharge_pages(old, nr_pages);
+		/* Flush complete pages back to the page stock */
+		if (nr_pages) {
+			struct mem_cgroup *memcg;
+
+			memcg = get_mem_cgroup_from_objcg(objcg);
+			mem_cgroup_kmem_record(memcg, -nr_pages);
+			__refill_stock(stock, memcg, nr_pages);
+			css_put(&memcg->css);
+		}
 
 		/*
 		 * The leftover is flushed to the centralized per-memcg value.

Re: [PATCH v2 1/4] mm/memcg: Revert ("mm/memcg: optimize user context object stock access")

[Johannes Weiner]

On Fri, Feb 11, 2022 at 11:35:34PM +0100, Sebastian Andrzej Siewior wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> The optimisation is based on a micro benchmark where local_irq_save() is
> more expensive than a preempt_disable(). There is no evidence that it is
> visible in a real-world workload and there are CPUs where the opposite is
> true (local_irq_save() is cheaper than preempt_disable()).
> 
> Based on micro benchmarks, the optimisation makes sense on PREEMPT_NONE
> where preempt_disable() is optimized away. There is no improvement with
> PREEMPT_DYNAMIC since the preemption counter is always available.
> 
> The optimization makes also the PREEMPT_RT integration more complicated
> since most of the assumption are not true on PREEMPT_RT.
> 
> Revert the optimisation since it complicates the PREEMPT_RT integration
> and the improvement is hardly visible.
> 
> [ bigeasy: Patch body around Michal's diff ]
> 
> Link: https://lore.kernel.org/all/YgOGkXXCrD%2F1k+p4@dhcp22.suse.cz
> Link: https://lkml.kernel.org/r/YdX+INO9gQje6d0S@linutronix.de
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Re: [PATCH v2 1/4] mm/memcg: Revert ("mm/memcg: optimize user context object stock access")

[Johannes Weiner]

On Fri, Feb 11, 2022 at 11:35:34PM +0100, Sebastian Andrzej Siewior wrote:
> From: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
> 
> The optimisation is based on a micro benchmark where local_irq_save() is
> more expensive than a preempt_disable(). There is no evidence that it is
> visible in a real-world workload and there are CPUs where the opposite is
> true (local_irq_save() is cheaper than preempt_disable()).
> 
> Based on micro benchmarks, the optimisation makes sense on PREEMPT_NONE
> where preempt_disable() is optimized away. There is no improvement with
> PREEMPT_DYNAMIC since the preemption counter is always available.
> 
> The optimization makes also the PREEMPT_RT integration more complicated
> since most of the assumption are not true on PREEMPT_RT.
> 
> Revert the optimisation since it complicates the PREEMPT_RT integration
> and the improvement is hardly visible.
> 
> [ bigeasy: Patch body around Michal's diff ]
> 
> Link: https://lore.kernel.org/all/YgOGkXXCrD%2F1k+p4-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org
> Link: https://lkml.kernel.org/r/YdX+INO9gQje6d0S-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org
> Signed-off-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>

Acked-by: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>

Re: [PATCH v2 2/4] mm/memcg: Disable threshold event handlers on PREEMPT_RT

[Johannes Weiner]

On Fri, Feb 11, 2022 at 11:35:35PM +0100, Sebastian Andrzej Siewior wrote:
> During the integration of PREEMPT_RT support, the code flow around
> memcg_check_events() resulted in `twisted code'. Moving the code around
> and avoiding then would then lead to an additional local-irq-save
> section within memcg_check_events(). While looking better, it adds a
> local-irq-save section to code flow which is usually within an
> local-irq-off block on non-PREEMPT_RT configurations.
> 
> The threshold event handler is a deprecated memcg v1 feature. Instead of
> trying to get it to work under PREEMPT_RT just disable it. There should
> be no users on PREEMPT_RT. From that perspective it makes even less
> sense to get it to work under PREEMPT_RT while having zero users.
> 
> Make memory.soft_limit_in_bytes and cgroup.event_control return
> -EOPNOTSUPP on PREEMPT_RT. Make an empty memcg_check_events() and
> memcg_write_event_control() which return only -EOPNOTSUPP on PREEMPT_RT.
> Document that the two knobs are disabled on PREEMPT_RT.
> 
> Suggested-by: Michal Hocko <mhocko@kernel.org>
> Suggested-by: Michal Koutný <mkoutny@suse.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

Re: [PATCH v2 2/4] mm/memcg: Disable threshold event handlers on PREEMPT_RT

[Johannes Weiner]

On Fri, Feb 11, 2022 at 11:35:35PM +0100, Sebastian Andrzej Siewior wrote:
> During the integration of PREEMPT_RT support, the code flow around
> memcg_check_events() resulted in `twisted code'. Moving the code around
> and avoiding then would then lead to an additional local-irq-save
> section within memcg_check_events(). While looking better, it adds a
> local-irq-save section to code flow which is usually within an
> local-irq-off block on non-PREEMPT_RT configurations.
> 
> The threshold event handler is a deprecated memcg v1 feature. Instead of
> trying to get it to work under PREEMPT_RT just disable it. There should
> be no users on PREEMPT_RT. From that perspective it makes even less
> sense to get it to work under PREEMPT_RT while having zero users.
> 
> Make memory.soft_limit_in_bytes and cgroup.event_control return
> -EOPNOTSUPP on PREEMPT_RT. Make an empty memcg_check_events() and
> memcg_write_event_control() which return only -EOPNOTSUPP on PREEMPT_RT.
> Document that the two knobs are disabled on PREEMPT_RT.
> 
> Suggested-by: Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Suggested-by: Michal Koutný <mkoutny-IBi9RG/b67k@public.gmane.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>

Acked-by: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>

Re: [PATCH v2 3/4] mm/memcg: Protect per-CPU counter by disabling preemption on PREEMPT_RT where needed.

[Johannes Weiner]

On Fri, Feb 11, 2022 at 11:35:36PM +0100, Sebastian Andrzej Siewior wrote:
> The per-CPU counter are modified with the non-atomic modifier. The
> consistency is ensured by disabling interrupts for the update.
> On non PREEMPT_RT configuration this works because acquiring a
> spinlock_t typed lock with the _irq() suffix disables interrupts. On
> PREEMPT_RT configurations the RMW operation can be interrupted.
> 
> Another problem is that mem_cgroup_swapout() expects to be invoked with
> disabled interrupts because the caller has to acquire a spinlock_t which
> is acquired with disabled interrupts. Since spinlock_t never disables
> interrupts on PREEMPT_RT the interrupts are never disabled at this
> point.
> 
> The code is never called from in_irq() context on PREEMPT_RT therefore
> disabling preemption during the update is sufficient on PREEMPT_RT.
> The sections which explicitly disable interrupts can remain on
> PREEMPT_RT because the sections remain short and they don't involve
> sleeping locks (memcg_check_events() is doing nothing on PREEMPT_RT).
> 
> Disable preemption during update of the per-CPU variables which do not
> explicitly disable interrupts.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  mm/memcontrol.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c1caa662946dc..466466f285cea 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -705,6 +705,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
>  	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
>  	memcg = pn->memcg;
>  
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +		preempt_disable();
>  	/* Update memcg */
>  	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
>  
> @@ -712,6 +714,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
>  	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
>  
>  	memcg_rstat_updated(memcg, val);
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +		preempt_enable();
>  }

I notice you didn't annoate __mod_memcg_state(). I suppose that is
because it's called with explicit local_irq_disable(), and that
disables preemption on rt? And you only need another preempt_disable()
for stacks that rely on coming from spin_lock_irq(save)?

That makes sense, but it's difficult to maintain. It'll easily break
if somebody adds more memory accounting sites that may also rely on an
irq-disabled spinlock somewhere.

So better to make this an unconditional locking protocol:

static void memcg_stats_lock(void)
{
#ifdef CONFIG_PREEMPT_RT
	preempt_disable();
#else
	VM_BUG_ON(!irqs_disabled());
#endif
}

static void memcg_stats_unlock(void)
{
#ifdef CONFIG_PREEMPT_RT
	preempt_enable();
#endif
}

and always use these around the counter updates.

Re: [PATCH v2 3/4] mm/memcg: Protect per-CPU counter by disabling preemption on PREEMPT_RT where needed.

[Johannes Weiner]

On Fri, Feb 11, 2022 at 11:35:36PM +0100, Sebastian Andrzej Siewior wrote:
> The per-CPU counter are modified with the non-atomic modifier. The
> consistency is ensured by disabling interrupts for the update.
> On non PREEMPT_RT configuration this works because acquiring a
> spinlock_t typed lock with the _irq() suffix disables interrupts. On
> PREEMPT_RT configurations the RMW operation can be interrupted.
> 
> Another problem is that mem_cgroup_swapout() expects to be invoked with
> disabled interrupts because the caller has to acquire a spinlock_t which
> is acquired with disabled interrupts. Since spinlock_t never disables
> interrupts on PREEMPT_RT the interrupts are never disabled at this
> point.
> 
> The code is never called from in_irq() context on PREEMPT_RT therefore
> disabling preemption during the update is sufficient on PREEMPT_RT.
> The sections which explicitly disable interrupts can remain on
> PREEMPT_RT because the sections remain short and they don't involve
> sleeping locks (memcg_check_events() is doing nothing on PREEMPT_RT).
> 
> Disable preemption during update of the per-CPU variables which do not
> explicitly disable interrupts.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
> ---
>  mm/memcontrol.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c1caa662946dc..466466f285cea 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -705,6 +705,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
>  	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
>  	memcg = pn->memcg;
>  
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +		preempt_disable();
>  	/* Update memcg */
>  	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
>  
> @@ -712,6 +714,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
>  	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
>  
>  	memcg_rstat_updated(memcg, val);
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +		preempt_enable();
>  }

I notice you didn't annoate __mod_memcg_state(). I suppose that is
because it's called with explicit local_irq_disable(), and that
disables preemption on rt? And you only need another preempt_disable()
for stacks that rely on coming from spin_lock_irq(save)?

That makes sense, but it's difficult to maintain. It'll easily break
if somebody adds more memory accounting sites that may also rely on an
irq-disabled spinlock somewhere.

So better to make this an unconditional locking protocol:

static void memcg_stats_lock(void)
{
#ifdef CONFIG_PREEMPT_RT
	preempt_disable();
#else
	VM_BUG_ON(!irqs_disabled());
#endif
}

static void memcg_stats_unlock(void)
{
#ifdef CONFIG_PREEMPT_RT
	preempt_enable();
#endif
}

and always use these around the counter updates.

Re: [PATCH v2 1/4] mm/memcg: Revert ("mm/memcg: optimize user context object stock access")

[Roman Gushchin]

On Fri, Feb 11, 2022 at 11:35:34PM +0100, Sebastian Andrzej Siewior wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> The optimisation is based on a micro benchmark where local_irq_save() is
> more expensive than a preempt_disable(). There is no evidence that it is
> visible in a real-world workload and there are CPUs where the opposite is
> true (local_irq_save() is cheaper than preempt_disable()).
> 
> Based on micro benchmarks, the optimisation makes sense on PREEMPT_NONE
> where preempt_disable() is optimized away. There is no improvement with
> PREEMPT_DYNAMIC since the preemption counter is always available.
> 
> The optimization makes also the PREEMPT_RT integration more complicated
> since most of the assumption are not true on PREEMPT_RT.
> 
> Revert the optimisation since it complicates the PREEMPT_RT integration
> and the improvement is hardly visible.
> 
> [ bigeasy: Patch body around Michal's diff ]
> 
> Link: https://lore.kernel.org/all/YgOGkXXCrD%2F1k+p4@dhcp22.suse.cz
> Link: https://lkml.kernel.org/r/YdX+INO9gQje6d0S@linutronix.de
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Acked-by: Roman Gushchin <guro@fb.com>

Thanks!

Re: [PATCH v2 1/4] mm/memcg: Revert ("mm/memcg: optimize user context object stock access")

[Roman Gushchin]

On Fri, Feb 11, 2022 at 11:35:34PM +0100, Sebastian Andrzej Siewior wrote:
> From: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
> 
> The optimisation is based on a micro benchmark where local_irq_save() is
> more expensive than a preempt_disable(). There is no evidence that it is
> visible in a real-world workload and there are CPUs where the opposite is
> true (local_irq_save() is cheaper than preempt_disable()).
> 
> Based on micro benchmarks, the optimisation makes sense on PREEMPT_NONE
> where preempt_disable() is optimized away. There is no improvement with
> PREEMPT_DYNAMIC since the preemption counter is always available.
> 
> The optimization makes also the PREEMPT_RT integration more complicated
> since most of the assumption are not true on PREEMPT_RT.
> 
> Revert the optimisation since it complicates the PREEMPT_RT integration
> and the improvement is hardly visible.
> 
> [ bigeasy: Patch body around Michal's diff ]
> 
> Link: https://lore.kernel.org/all/YgOGkXXCrD%2F1k+p4-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org
> Link: https://lkml.kernel.org/r/YdX+INO9gQje6d0S-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org
> Signed-off-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>

Acked-by: Roman Gushchin <guro-b10kYP2dOMg@public.gmane.org>

Thanks!

Re: [PATCH v2 2/4] mm/memcg: Disable threshold event handlers on PREEMPT_RT

[Roman Gushchin]

On Fri, Feb 11, 2022 at 11:35:35PM +0100, Sebastian Andrzej Siewior wrote:
> During the integration of PREEMPT_RT support, the code flow around
> memcg_check_events() resulted in `twisted code'. Moving the code around
> and avoiding then would then lead to an additional local-irq-save
> section within memcg_check_events(). While looking better, it adds a
> local-irq-save section to code flow which is usually within an
> local-irq-off block on non-PREEMPT_RT configurations.
> 
> The threshold event handler is a deprecated memcg v1 feature. Instead of
> trying to get it to work under PREEMPT_RT just disable it. There should
> be no users on PREEMPT_RT. From that perspective it makes even less
> sense to get it to work under PREEMPT_RT while having zero users.
> 
> Make memory.soft_limit_in_bytes and cgroup.event_control return
> -EOPNOTSUPP on PREEMPT_RT. Make an empty memcg_check_events() and
> memcg_write_event_control() which return only -EOPNOTSUPP on PREEMPT_RT.
> Document that the two knobs are disabled on PREEMPT_RT.
> 
> Suggested-by: Michal Hocko <mhocko@kernel.org>
> Suggested-by: Michal Koutný <mkoutny@suse.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Acked-by: Roman Gushchin <guro@fb.com>

Re: [PATCH v2 2/4] mm/memcg: Disable threshold event handlers on PREEMPT_RT

[Roman Gushchin]

On Fri, Feb 11, 2022 at 11:35:35PM +0100, Sebastian Andrzej Siewior wrote:
> During the integration of PREEMPT_RT support, the code flow around
> memcg_check_events() resulted in `twisted code'. Moving the code around
> and avoiding then would then lead to an additional local-irq-save
> section within memcg_check_events(). While looking better, it adds a
> local-irq-save section to code flow which is usually within an
> local-irq-off block on non-PREEMPT_RT configurations.
> 
> The threshold event handler is a deprecated memcg v1 feature. Instead of
> trying to get it to work under PREEMPT_RT just disable it. There should
> be no users on PREEMPT_RT. From that perspective it makes even less
> sense to get it to work under PREEMPT_RT while having zero users.
> 
> Make memory.soft_limit_in_bytes and cgroup.event_control return
> -EOPNOTSUPP on PREEMPT_RT. Make an empty memcg_check_events() and
> memcg_write_event_control() which return only -EOPNOTSUPP on PREEMPT_RT.
> Document that the two knobs are disabled on PREEMPT_RT.
> 
> Suggested-by: Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Suggested-by: Michal Koutný <mkoutny-IBi9RG/b67k@public.gmane.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>

Acked-by: Roman Gushchin <guro-b10kYP2dOMg@public.gmane.org>

Re: [PATCH v2 3/4] mm/memcg: Protect per-CPU counter by disabling preemption on PREEMPT_RT where needed.

[Roman Gushchin]

On Mon, Feb 14, 2022 at 11:46:00AM -0500, Johannes Weiner wrote:
> On Fri, Feb 11, 2022 at 11:35:36PM +0100, Sebastian Andrzej Siewior wrote:
> > The per-CPU counter are modified with the non-atomic modifier. The
> > consistency is ensured by disabling interrupts for the update.
> > On non PREEMPT_RT configuration this works because acquiring a
> > spinlock_t typed lock with the _irq() suffix disables interrupts. On
> > PREEMPT_RT configurations the RMW operation can be interrupted.
> > 
> > Another problem is that mem_cgroup_swapout() expects to be invoked with
> > disabled interrupts because the caller has to acquire a spinlock_t which
> > is acquired with disabled interrupts. Since spinlock_t never disables
> > interrupts on PREEMPT_RT the interrupts are never disabled at this
> > point.
> > 
> > The code is never called from in_irq() context on PREEMPT_RT therefore
> > disabling preemption during the update is sufficient on PREEMPT_RT.
> > The sections which explicitly disable interrupts can remain on
> > PREEMPT_RT because the sections remain short and they don't involve
> > sleeping locks (memcg_check_events() is doing nothing on PREEMPT_RT).
> > 
> > Disable preemption during update of the per-CPU variables which do not
> > explicitly disable interrupts.
> > 
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > ---
> >  mm/memcontrol.c | 21 +++++++++++++++++++--
> >  1 file changed, 19 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index c1caa662946dc..466466f285cea 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -705,6 +705,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> >  	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> >  	memcg = pn->memcg;
> >  
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +		preempt_disable();
> >  	/* Update memcg */
> >  	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
> >  
> > @@ -712,6 +714,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> >  	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
> >  
> >  	memcg_rstat_updated(memcg, val);
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +		preempt_enable();
> >  }
> 
> I notice you didn't annoate __mod_memcg_state(). I suppose that is
> because it's called with explicit local_irq_disable(), and that
> disables preemption on rt? And you only need another preempt_disable()
> for stacks that rely on coming from spin_lock_irq(save)?
> 
> That makes sense, but it's difficult to maintain. It'll easily break
> if somebody adds more memory accounting sites that may also rely on an
> irq-disabled spinlock somewhere.
> 
> So better to make this an unconditional locking protocol:
> 
> static void memcg_stats_lock(void)
> {
> #ifdef CONFIG_PREEMPT_RT
> 	preempt_disable();
> #else
> 	VM_BUG_ON(!irqs_disabled());
> #endif
> }
> 
> static void memcg_stats_unlock(void)
> {
> #ifdef CONFIG_PREEMPT_RT
> 	preempt_enable();
> #endif
> }
> 
> and always use these around the counter updates.

Thanks, Johannes, this looks really good to me. The code is already quite
complicated, the suggested locking protocol makes it easier to read and
support.

Otherwise the patch looks good to me. Sebastian, please, feel to add
Acked-by: Roman Gushchin <guro@fb.com> after incorporating Johannes's
suggestion.

Thanks!

Re: [PATCH v2 3/4] mm/memcg: Protect per-CPU counter by disabling preemption on PREEMPT_RT where needed.

[Roman Gushchin]

On Mon, Feb 14, 2022 at 11:46:00AM -0500, Johannes Weiner wrote:
> On Fri, Feb 11, 2022 at 11:35:36PM +0100, Sebastian Andrzej Siewior wrote:
> > The per-CPU counter are modified with the non-atomic modifier. The
> > consistency is ensured by disabling interrupts for the update.
> > On non PREEMPT_RT configuration this works because acquiring a
> > spinlock_t typed lock with the _irq() suffix disables interrupts. On
> > PREEMPT_RT configurations the RMW operation can be interrupted.
> > 
> > Another problem is that mem_cgroup_swapout() expects to be invoked with
> > disabled interrupts because the caller has to acquire a spinlock_t which
> > is acquired with disabled interrupts. Since spinlock_t never disables
> > interrupts on PREEMPT_RT the interrupts are never disabled at this
> > point.
> > 
> > The code is never called from in_irq() context on PREEMPT_RT therefore
> > disabling preemption during the update is sufficient on PREEMPT_RT.
> > The sections which explicitly disable interrupts can remain on
> > PREEMPT_RT because the sections remain short and they don't involve
> > sleeping locks (memcg_check_events() is doing nothing on PREEMPT_RT).
> > 
> > Disable preemption during update of the per-CPU variables which do not
> > explicitly disable interrupts.
> > 
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
> > ---
> >  mm/memcontrol.c | 21 +++++++++++++++++++--
> >  1 file changed, 19 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index c1caa662946dc..466466f285cea 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -705,6 +705,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> >  	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> >  	memcg = pn->memcg;
> >  
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +		preempt_disable();
> >  	/* Update memcg */
> >  	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
> >  
> > @@ -712,6 +714,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> >  	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
> >  
> >  	memcg_rstat_updated(memcg, val);
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +		preempt_enable();
> >  }
> 
> I notice you didn't annoate __mod_memcg_state(). I suppose that is
> because it's called with explicit local_irq_disable(), and that
> disables preemption on rt? And you only need another preempt_disable()
> for stacks that rely on coming from spin_lock_irq(save)?
> 
> That makes sense, but it's difficult to maintain. It'll easily break
> if somebody adds more memory accounting sites that may also rely on an
> irq-disabled spinlock somewhere.
> 
> So better to make this an unconditional locking protocol:
> 
> static void memcg_stats_lock(void)
> {
> #ifdef CONFIG_PREEMPT_RT
> 	preempt_disable();
> #else
> 	VM_BUG_ON(!irqs_disabled());
> #endif
> }
> 
> static void memcg_stats_unlock(void)
> {
> #ifdef CONFIG_PREEMPT_RT
> 	preempt_enable();
> #endif
> }
> 
> and always use these around the counter updates.

Thanks, Johannes, this looks really good to me. The code is already quite
complicated, the suggested locking protocol makes it easier to read and
support.

Otherwise the patch looks good to me. Sebastian, please, feel to add
Acked-by: Roman Gushchin <guro-b10kYP2dOMg@public.gmane.org> after incorporating Johannes's
suggestion.

Thanks!

Re: [PATCH v2 3/4] mm/memcg: Protect per-CPU counter by disabling preemption on PREEMPT_RT where needed.

[Sebastian Andrzej Siewior]

On 2022-02-14 11:46:00 [-0500], Johannes Weiner wrote:
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index c1caa662946dc..466466f285cea 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -705,6 +705,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> >  	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> >  	memcg = pn->memcg;
> >  
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +		preempt_disable();
> >  	/* Update memcg */
> >  	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
> >  
> > @@ -712,6 +714,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> >  	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
> >  
> >  	memcg_rstat_updated(memcg, val);
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +		preempt_enable();
> >  }
> 
> I notice you didn't annoate __mod_memcg_state(). I suppose that is
> because it's called with explicit local_irq_disable(), and that
> disables preemption on rt? And you only need another preempt_disable()
> for stacks that rely on coming from spin_lock_irq(save)?

Correct. The code is not used in_hardirq() on PREEMPT_RT so
preempt_disable() is sufficient. I didn't bother to replace all
local_irq_save() with preempt_disable() since it is probably not worth
it.
And yes: spin_lock_irq() does not disable interrupts so I need something
here to ensure that the RMW operation is not interrupted.

> That makes sense, but it's difficult to maintain. It'll easily break
> if somebody adds more memory accounting sites that may also rely on an
> irq-disabled spinlock somewhere.
> 
> So better to make this an unconditional locking protocol:
> 
> static void memcg_stats_lock(void)
> {
> #ifdef CONFIG_PREEMPT_RT
> 	preempt_disable();
> #else
> 	VM_BUG_ON(!irqs_disabled());
> #endif
> }
> 
> static void memcg_stats_unlock(void)
> {
> #ifdef CONFIG_PREEMPT_RT
> 	preempt_enable();
> #endif
> }
> 
> and always use these around the counter updates.

Something like the following perhaps? I didn't add anything to
__mod_memcg_state() since it has no users besides the one which does
local_irq_save().

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c1caa662946dc..69130a5fe3d51 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -629,6 +629,28 @@ static DEFINE_SPINLOCK(stats_flush_lock);
 static DEFINE_PER_CPU(unsigned int, stats_updates);
 static atomic_t stats_flush_threshold = ATOMIC_INIT(0);
 
+/*
+ * Accessors to ensure that preemption is disabled on PREEMPT_RT because it can
+ * not rely on this as part of an acquired spinlock_t lock. These functions are
+ * never used in hardirq context on PREEMPT_RT and therefore disabling preemtion
+ * is sufficient.
+ */
+static void memcg_stats_lock(void)
+{
+#ifdef CONFIG_PREEMPT_RT
+      preempt_disable();
+#else
+      VM_BUG_ON(!irqs_disabled());
+#endif
+}
+
+static void memcg_stats_unlock(void)
+{
+#ifdef CONFIG_PREEMPT_RT
+      preempt_enable();
+#endif
+}
+
 static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
 {
 	unsigned int x;
@@ -705,6 +727,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
 	memcg = pn->memcg;
 
+	memcg_stats_lock();
 	/* Update memcg */
 	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
 
@@ -712,6 +735,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
 
 	memcg_rstat_updated(memcg, val);
+	memcg_stats_unlock();
 }
 
 /**
@@ -794,8 +818,10 @@ void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
 	if (mem_cgroup_disabled())
 		return;
 
+	memcg_stats_lock();
 	__this_cpu_add(memcg->vmstats_percpu->events[idx], count);
 	memcg_rstat_updated(memcg, count);
+	memcg_stats_unlock();
 }
 
 static unsigned long memcg_events(struct mem_cgroup *memcg, int event)
@@ -7149,8 +7175,9 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
 	 * important here to have the interrupts disabled because it is the
 	 * only synchronisation we have for updating the per-CPU variables.
 	 */
-	VM_BUG_ON(!irqs_disabled());
+	memcg_stats_lock();
 	mem_cgroup_charge_statistics(memcg, -nr_entries);
+	memcg_stats_unlock();
 	memcg_check_events(memcg, page_to_nid(page));
 
 	css_put(&memcg->css);
-- 
2.34.1

Re: [PATCH v2 3/4] mm/memcg: Protect per-CPU counter by disabling preemption on PREEMPT_RT where needed.

[Sebastian Andrzej Siewior]

On 2022-02-14 11:46:00 [-0500], Johannes Weiner wrote:
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index c1caa662946dc..466466f285cea 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -705,6 +705,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> >  	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> >  	memcg = pn->memcg;
> >  
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +		preempt_disable();
> >  	/* Update memcg */
> >  	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
> >  
> > @@ -712,6 +714,8 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
> >  	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
> >  
> >  	memcg_rstat_updated(memcg, val);
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > +		preempt_enable();
> >  }
> 
> I notice you didn't annoate __mod_memcg_state(). I suppose that is
> because it's called with explicit local_irq_disable(), and that
> disables preemption on rt? And you only need another preempt_disable()
> for stacks that rely on coming from spin_lock_irq(save)?

Correct. The code is not used in_hardirq() on PREEMPT_RT so
preempt_disable() is sufficient. I didn't bother to replace all
local_irq_save() with preempt_disable() since it is probably not worth
it.
And yes: spin_lock_irq() does not disable interrupts so I need something
here to ensure that the RMW operation is not interrupted.

> That makes sense, but it's difficult to maintain. It'll easily break
> if somebody adds more memory accounting sites that may also rely on an
> irq-disabled spinlock somewhere.
> 
> So better to make this an unconditional locking protocol:
> 
> static void memcg_stats_lock(void)
> {
> #ifdef CONFIG_PREEMPT_RT
> 	preempt_disable();
> #else
> 	VM_BUG_ON(!irqs_disabled());
> #endif
> }
> 
> static void memcg_stats_unlock(void)
> {
> #ifdef CONFIG_PREEMPT_RT
> 	preempt_enable();
> #endif
> }
> 
> and always use these around the counter updates.

Something like the following perhaps? I didn't add anything to
__mod_memcg_state() since it has no users besides the one which does
local_irq_save().

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c1caa662946dc..69130a5fe3d51 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -629,6 +629,28 @@ static DEFINE_SPINLOCK(stats_flush_lock);
 static DEFINE_PER_CPU(unsigned int, stats_updates);
 static atomic_t stats_flush_threshold = ATOMIC_INIT(0);
 
+/*
+ * Accessors to ensure that preemption is disabled on PREEMPT_RT because it can
+ * not rely on this as part of an acquired spinlock_t lock. These functions are
+ * never used in hardirq context on PREEMPT_RT and therefore disabling preemtion
+ * is sufficient.
+ */
+static void memcg_stats_lock(void)
+{
+#ifdef CONFIG_PREEMPT_RT
+      preempt_disable();
+#else
+      VM_BUG_ON(!irqs_disabled());
+#endif
+}
+
+static void memcg_stats_unlock(void)
+{
+#ifdef CONFIG_PREEMPT_RT
+      preempt_enable();
+#endif
+}
+
 static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
 {
 	unsigned int x;
@@ -705,6 +727,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
 	memcg = pn->memcg;
 
+	memcg_stats_lock();
 	/* Update memcg */
 	__this_cpu_add(memcg->vmstats_percpu->state[idx], val);
 
@@ -712,6 +735,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	__this_cpu_add(pn->lruvec_stats_percpu->state[idx], val);
 
 	memcg_rstat_updated(memcg, val);
+	memcg_stats_unlock();
 }
 
 /**
@@ -794,8 +818,10 @@ void __count_memcg_events(struct mem_cgroup *memcg, enum vm_event_item idx,
 	if (mem_cgroup_disabled())
 		return;
 
+	memcg_stats_lock();
 	__this_cpu_add(memcg->vmstats_percpu->events[idx], count);
 	memcg_rstat_updated(memcg, count);
+	memcg_stats_unlock();
 }
 
 static unsigned long memcg_events(struct mem_cgroup *memcg, int event)
@@ -7149,8 +7175,9 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
 	 * important here to have the interrupts disabled because it is the
 	 * only synchronisation we have for updating the per-CPU variables.
 	 */
-	VM_BUG_ON(!irqs_disabled());
+	memcg_stats_lock();
 	mem_cgroup_charge_statistics(memcg, -nr_entries);
+	memcg_stats_unlock();
 	memcg_check_events(memcg, page_to_nid(page));
 
 	css_put(&memcg->css);
-- 
2.34.1

Re: [PATCH v2 4/4] mm/memcg: Protect memcg_stock with a local_lock_t

[Sebastian Andrzej Siewior]

On 2022-02-14 11:23:11 [-0500], Johannes Weiner wrote:
> Hi Sebastian,
Hi Johannes,

> This is a bit dubious in terms of layering. It's an objcg operation,
> but what's "locked" isn't the objcg, it's the underlying stock. That
> function then looks it up again, even though we have it right there.
> 
> You can open-code it and factor out the stock operation instead, and
> it makes things much simpler and clearer.
> 
> I.e. something like this (untested!):

This then:

------>8------

From: Johannes Weiner <hannes@cmpxchg.org>
Date: Wed, 16 Feb 2022 13:25:49 +0100
Subject: [PATCH] mm/memcg: Opencode the inner part of
 obj_cgroup_uncharge_pages() in drain_obj_stock()

Provide the inner part of refill_stock() as __refill_stock() without
disabling interrupts. This eases the integration of local_lock_t where
recursive locking must be avoided.
Open code obj_cgroup_uncharge_pages() in drain_obj_stock() and use
__refill_stock(). The caller of drain_obj_stock() already disables
interrupts.

[bigeasy: Patch body around Johannes' diff ]

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 mm/memcontrol.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 69130a5fe3d51..f574f2e1cc399 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2227,12 +2227,9 @@ static void drain_local_stock(struct work_struct *dummy)
  * Cache charges(val) to local per_cpu area.
  * This will be consumed by consume_stock() function, later.
  */
-static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
+static void __refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 {
 	struct memcg_stock_pcp *stock;
-	unsigned long flags;
-
-	local_irq_save(flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
 	if (stock->cached != memcg) { /* reset if necessary */
@@ -2244,7 +2241,14 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 
 	if (stock->nr_pages > MEMCG_CHARGE_BATCH)
 		drain_stock(stock);
+}
 
+static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+	__refill_stock(memcg, nr_pages);
 	local_irq_restore(flags);
 }
 
@@ -3151,8 +3155,17 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
 		unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
 		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
 
-		if (nr_pages)
-			obj_cgroup_uncharge_pages(old, nr_pages);
+		if (nr_pages) {
+			struct mem_cgroup *memcg;
+
+			memcg = get_mem_cgroup_from_objcg(old);
+
+			if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
+				page_counter_uncharge(&memcg->kmem, nr_pages);
+			__refill_stock(memcg, nr_pages);
+
+			css_put(&memcg->css);
+		}
 
 		/*
 		 * The leftover is flushed to the centralized per-memcg value.
-- 
2.34.1

Re: [PATCH v2 4/4] mm/memcg: Protect memcg_stock with a local_lock_t

[Sebastian Andrzej Siewior]

On 2022-02-14 11:23:11 [-0500], Johannes Weiner wrote:
> Hi Sebastian,
Hi Johannes,

> This is a bit dubious in terms of layering. It's an objcg operation,
> but what's "locked" isn't the objcg, it's the underlying stock. That
> function then looks it up again, even though we have it right there.
> 
> You can open-code it and factor out the stock operation instead, and
> it makes things much simpler and clearer.
> 
> I.e. something like this (untested!):

This then:

------>8------

From: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
Date: Wed, 16 Feb 2022 13:25:49 +0100
Subject: [PATCH] mm/memcg: Opencode the inner part of
 obj_cgroup_uncharge_pages() in drain_obj_stock()

Provide the inner part of refill_stock() as __refill_stock() without
disabling interrupts. This eases the integration of local_lock_t where
recursive locking must be avoided.
Open code obj_cgroup_uncharge_pages() in drain_obj_stock() and use
__refill_stock(). The caller of drain_obj_stock() already disables
interrupts.

[bigeasy: Patch body around Johannes' diff ]

Signed-off-by: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
---
 mm/memcontrol.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 69130a5fe3d51..f574f2e1cc399 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2227,12 +2227,9 @@ static void drain_local_stock(struct work_struct *dummy)
  * Cache charges(val) to local per_cpu area.
  * This will be consumed by consume_stock() function, later.
  */
-static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
+static void __refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 {
 	struct memcg_stock_pcp *stock;
-	unsigned long flags;
-
-	local_irq_save(flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
 	if (stock->cached != memcg) { /* reset if necessary */
@@ -2244,7 +2241,14 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 
 	if (stock->nr_pages > MEMCG_CHARGE_BATCH)
 		drain_stock(stock);
+}
 
+static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+	__refill_stock(memcg, nr_pages);
 	local_irq_restore(flags);
 }
 
@@ -3151,8 +3155,17 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
 		unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
 		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
 
-		if (nr_pages)
-			obj_cgroup_uncharge_pages(old, nr_pages);
+		if (nr_pages) {
+			struct mem_cgroup *memcg;
+
+			memcg = get_mem_cgroup_from_objcg(old);
+
+			if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
+				page_counter_uncharge(&memcg->kmem, nr_pages);
+			__refill_stock(memcg, nr_pages);
+
+			css_put(&memcg->css);
+		}
 
 		/*
 		 * The leftover is flushed to the centralized per-memcg value.
-- 
2.34.1

Re: [PATCH v2 4/4] mm/memcg: Protect memcg_stock with a local_lock_t

[Johannes Weiner]

On Wed, Feb 16, 2022 at 04:51:14PM +0100, Sebastian Andrzej Siewior wrote:
> On 2022-02-14 11:23:11 [-0500], Johannes Weiner wrote:
> > Hi Sebastian,
> Hi Johannes,
> 
> > This is a bit dubious in terms of layering. It's an objcg operation,
> > but what's "locked" isn't the objcg, it's the underlying stock. That
> > function then looks it up again, even though we have it right there.
> > 
> > You can open-code it and factor out the stock operation instead, and
> > it makes things much simpler and clearer.
> > 
> > I.e. something like this (untested!):
> 
> This then:
> 
> ------>8------
> 
> From: Johannes Weiner <hannes@cmpxchg.org>
> Date: Wed, 16 Feb 2022 13:25:49 +0100
> Subject: [PATCH] mm/memcg: Opencode the inner part of
>  obj_cgroup_uncharge_pages() in drain_obj_stock()
> 
> Provide the inner part of refill_stock() as __refill_stock() without
> disabling interrupts. This eases the integration of local_lock_t where
> recursive locking must be avoided.
> Open code obj_cgroup_uncharge_pages() in drain_obj_stock() and use
> __refill_stock(). The caller of drain_obj_stock() already disables
> interrupts.
> 
> [bigeasy: Patch body around Johannes' diff ]
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

I thought you'd fold it into yours, but separate patch works too,
thanks!

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

One important note, though:

> @@ -3151,8 +3155,17 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
>  		unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
>  		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
>  
> -		if (nr_pages)
> -			obj_cgroup_uncharge_pages(old, nr_pages);
> +		if (nr_pages) {
> +			struct mem_cgroup *memcg;
> +
> +			memcg = get_mem_cgroup_from_objcg(old);
> +
> +			if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
> +				page_counter_uncharge(&memcg->kmem, nr_pages);
> +			__refill_stock(memcg, nr_pages);

This doesn't take "memcg: add per-memcg total kernel memory stat"
queued in -mm into account and so will break kmem accounting.

Make sure to rebase the patches to the -mm tree before sending it
out. You can find it here: https://github.com/hnaz/linux-mm

Re: [PATCH v2 4/4] mm/memcg: Protect memcg_stock with a local_lock_t

[Johannes Weiner]

On Wed, Feb 16, 2022 at 04:51:14PM +0100, Sebastian Andrzej Siewior wrote:
> On 2022-02-14 11:23:11 [-0500], Johannes Weiner wrote:
> > Hi Sebastian,
> Hi Johannes,
> 
> > This is a bit dubious in terms of layering. It's an objcg operation,
> > but what's "locked" isn't the objcg, it's the underlying stock. That
> > function then looks it up again, even though we have it right there.
> > 
> > You can open-code it and factor out the stock operation instead, and
> > it makes things much simpler and clearer.
> > 
> > I.e. something like this (untested!):
> 
> This then:
> 
> ------>8------
> 
> From: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
> Date: Wed, 16 Feb 2022 13:25:49 +0100
> Subject: [PATCH] mm/memcg: Opencode the inner part of
>  obj_cgroup_uncharge_pages() in drain_obj_stock()
> 
> Provide the inner part of refill_stock() as __refill_stock() without
> disabling interrupts. This eases the integration of local_lock_t where
> recursive locking must be avoided.
> Open code obj_cgroup_uncharge_pages() in drain_obj_stock() and use
> __refill_stock(). The caller of drain_obj_stock() already disables
> interrupts.
> 
> [bigeasy: Patch body around Johannes' diff ]
> 
> Signed-off-by: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>

I thought you'd fold it into yours, but separate patch works too,
thanks!

Signed-off-by: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>

One important note, though:

> @@ -3151,8 +3155,17 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
>  		unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
>  		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
>  
> -		if (nr_pages)
> -			obj_cgroup_uncharge_pages(old, nr_pages);
> +		if (nr_pages) {
> +			struct mem_cgroup *memcg;
> +
> +			memcg = get_mem_cgroup_from_objcg(old);
> +
> +			if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
> +				page_counter_uncharge(&memcg->kmem, nr_pages);
> +			__refill_stock(memcg, nr_pages);

This doesn't take "memcg: add per-memcg total kernel memory stat"
queued in -mm into account and so will break kmem accounting.

Make sure to rebase the patches to the -mm tree before sending it
out. You can find it here: https://github.com/hnaz/linux-mm

Re: [PATCH v2 4/4] mm/memcg: Protect memcg_stock with a local_lock_t

[Sebastian Andrzej Siewior]

On 2022-02-16 13:08:55 [-0500], Johannes Weiner wrote:
> I thought you'd fold it into yours, but separate patch works too,
> thanks!
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Thank you.

> One important note, though:
> 
> > @@ -3151,8 +3155,17 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
> >  		unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
> >  		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
> >  
> > -		if (nr_pages)
> > -			obj_cgroup_uncharge_pages(old, nr_pages);
> > +		if (nr_pages) {
> > +			struct mem_cgroup *memcg;
> > +
> > +			memcg = get_mem_cgroup_from_objcg(old);
> > +
> > +			if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
> > +				page_counter_uncharge(&memcg->kmem, nr_pages);
> > +			__refill_stock(memcg, nr_pages);
> 
> This doesn't take "memcg: add per-memcg total kernel memory stat"
> queued in -mm into account and so will break kmem accounting.
>
> Make sure to rebase the patches to the -mm tree before sending it
> out. You can find it here: https://github.com/hnaz/linux-mm

Thank you, rebased on-top.

Sebastian

Re: [PATCH v2 4/4] mm/memcg: Protect memcg_stock with a local_lock_t

[Sebastian Andrzej Siewior]

On 2022-02-16 13:08:55 [-0500], Johannes Weiner wrote:
> I thought you'd fold it into yours, but separate patch works too,
> thanks!
> 
> Signed-off-by: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>

Thank you.

> One important note, though:
> 
> > @@ -3151,8 +3155,17 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
> >  		unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
> >  		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
> >  
> > -		if (nr_pages)
> > -			obj_cgroup_uncharge_pages(old, nr_pages);
> > +		if (nr_pages) {
> > +			struct mem_cgroup *memcg;
> > +
> > +			memcg = get_mem_cgroup_from_objcg(old);
> > +
> > +			if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
> > +				page_counter_uncharge(&memcg->kmem, nr_pages);
> > +			__refill_stock(memcg, nr_pages);
> 
> This doesn't take "memcg: add per-memcg total kernel memory stat"
> queued in -mm into account and so will break kmem accounting.
>
> Make sure to rebase the patches to the -mm tree before sending it
> out. You can find it here: https://github.com/hnaz/linux-mm

Thank you, rebased on-top.

Sebastian

Re: [PATCH v2 1/4] mm/memcg: Revert ("mm/memcg: optimize user context object stock access")

[kernel test robot]

[-- Attachment #1: Type: text/plain, Size: 6849 bytes --]

CC: kbuild-all(a)lists.01.org
In-Reply-To: <20220211223537.2175879-2-bigeasy@linutronix.de>
References: <20220211223537.2175879-2-bigeasy@linutronix.de>
TO: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
TO: cgroups(a)vger.kernel.org
TO: linux-mm(a)kvack.org
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Linux Memory Management List <linux-mm@kvack.org>
CC: Johannes Weiner <hannes@cmpxchg.org>
CC: Michal Hocko <mhocko@kernel.org>
CC: "Michal Koutný" <mkoutny@suse.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Vladimir Davydov <vdavydov.dev@gmail.com>
CC: Waiman Long <longman@redhat.com>
CC: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Hi Sebastian,

I love your patch! Perhaps something to improve:

[auto build test WARNING on v5.17-rc3]
[also build test WARNING on next-20220211]
[cannot apply to hnaz-mm/master tj-cgroup/for-next]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Sebastian-Andrzej-Siewior/mm-memcg-Address-PREEMPT_RT-problems-instead-of-disabling-it/20220212-063701
base:    dfd42facf1e4ada021b939b4e19c935dcdd55566
:::::: branch date: 11 hours ago
:::::: commit date: 11 hours ago
config: arc-randconfig-m031-20220211 (https://download.01.org/0day-ci/archive/20220212/202202121755.VaCaguQX-lkp(a)intel.com/config)
compiler: arc-elf-gcc (GCC) 11.2.0

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>

New smatch warnings:
mm/memcontrol.c:6826 uncharge_folio() error: uninitialized symbol 'objcg'.

Old smatch warnings:
arch/arc/include/asm/thread_info.h:65 current_thread_info() error: uninitialized symbol 'sp'.

vim +/objcg +6826 mm/memcontrol.c

747db954cab64c Johannes Weiner         2014-08-08  6779  
c4ed6ebfcb0929 Matthew Wilcox (Oracle  2021-06-29  6780) static void uncharge_folio(struct folio *folio, struct uncharge_gather *ug)
747db954cab64c Johannes Weiner         2014-08-08  6781  {
c4ed6ebfcb0929 Matthew Wilcox (Oracle  2021-06-29  6782) 	long nr_pages;
b4e0b68fbd9d1f Muchun Song             2021-04-29  6783  	struct mem_cgroup *memcg;
b4e0b68fbd9d1f Muchun Song             2021-04-29  6784  	struct obj_cgroup *objcg;
9f762dbe19b9f1 Johannes Weiner         2020-06-03  6785  
c4ed6ebfcb0929 Matthew Wilcox (Oracle  2021-06-29  6786) 	VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
747db954cab64c Johannes Weiner         2014-08-08  6787  
747db954cab64c Johannes Weiner         2014-08-08  6788  	/*
747db954cab64c Johannes Weiner         2014-08-08  6789  	 * Nobody should be changing or seriously looking at
c4ed6ebfcb0929 Matthew Wilcox (Oracle  2021-06-29  6790) 	 * folio memcg or objcg at this point, we have fully
c4ed6ebfcb0929 Matthew Wilcox (Oracle  2021-06-29  6791) 	 * exclusive access to the folio.
747db954cab64c Johannes Weiner         2014-08-08  6792  	 */
a6062be9ef6494 Michal Hocko            2022-02-11  6793  	if (folio_memcg_kmem(folio)) {
1b7e4464d43a48 Matthew Wilcox (Oracle  2021-06-28  6794) 		objcg = __folio_objcg(folio);
b4e0b68fbd9d1f Muchun Song             2021-04-29  6795  		/*
b4e0b68fbd9d1f Muchun Song             2021-04-29  6796  		 * This get matches the put at the end of the function and
b4e0b68fbd9d1f Muchun Song             2021-04-29  6797  		 * kmem pages do not hold memcg references anymore.
b4e0b68fbd9d1f Muchun Song             2021-04-29  6798  		 */
b4e0b68fbd9d1f Muchun Song             2021-04-29  6799  		memcg = get_mem_cgroup_from_objcg(objcg);
b4e0b68fbd9d1f Muchun Song             2021-04-29  6800  	} else {
1b7e4464d43a48 Matthew Wilcox (Oracle  2021-06-28  6801) 		memcg = __folio_memcg(folio);
b4e0b68fbd9d1f Muchun Song             2021-04-29  6802  	}
b4e0b68fbd9d1f Muchun Song             2021-04-29  6803  
b4e0b68fbd9d1f Muchun Song             2021-04-29  6804  	if (!memcg)
b4e0b68fbd9d1f Muchun Song             2021-04-29  6805  		return;
747db954cab64c Johannes Weiner         2014-08-08  6806  
b4e0b68fbd9d1f Muchun Song             2021-04-29  6807  	if (ug->memcg != memcg) {
a9d5adeeb4b2c7 Jérôme Glisse           2017-09-08  6808  		if (ug->memcg) {
a9d5adeeb4b2c7 Jérôme Glisse           2017-09-08  6809  			uncharge_batch(ug);
a9d5adeeb4b2c7 Jérôme Glisse           2017-09-08  6810  			uncharge_gather_clear(ug);
747db954cab64c Johannes Weiner         2014-08-08  6811  		}
b4e0b68fbd9d1f Muchun Song             2021-04-29  6812  		ug->memcg = memcg;
c4ed6ebfcb0929 Matthew Wilcox (Oracle  2021-06-29  6813) 		ug->nid = folio_nid(folio);
f1796544a0ca0f Michal Hocko            2020-09-04  6814  
f1796544a0ca0f Michal Hocko            2020-09-04  6815  		/* pairs with css_put in uncharge_batch */
b4e0b68fbd9d1f Muchun Song             2021-04-29  6816  		css_get(&memcg->css);
747db954cab64c Johannes Weiner         2014-08-08  6817  	}
747db954cab64c Johannes Weiner         2014-08-08  6818  
c4ed6ebfcb0929 Matthew Wilcox (Oracle  2021-06-29  6819) 	nr_pages = folio_nr_pages(folio);
9f762dbe19b9f1 Johannes Weiner         2020-06-03  6820  
a6062be9ef6494 Michal Hocko            2022-02-11  6821  	if (folio_memcg_kmem(folio)) {
b4e0b68fbd9d1f Muchun Song             2021-04-29  6822  		ug->nr_memory += nr_pages;
9f762dbe19b9f1 Johannes Weiner         2020-06-03  6823  		ug->nr_kmem += nr_pages;
b4e0b68fbd9d1f Muchun Song             2021-04-29  6824  
c4ed6ebfcb0929 Matthew Wilcox (Oracle  2021-06-29  6825) 		folio->memcg_data = 0;
b4e0b68fbd9d1f Muchun Song             2021-04-29 @6826  		obj_cgroup_put(objcg);
b4e0b68fbd9d1f Muchun Song             2021-04-29  6827  	} else {
b4e0b68fbd9d1f Muchun Song             2021-04-29  6828  		/* LRU pages aren't accounted at the root level */
b4e0b68fbd9d1f Muchun Song             2021-04-29  6829  		if (!mem_cgroup_is_root(memcg))
b4e0b68fbd9d1f Muchun Song             2021-04-29  6830  			ug->nr_memory += nr_pages;
18b2db3b038522 Roman Gushchin          2020-12-01  6831  		ug->pgpgout++;
747db954cab64c Johannes Weiner         2014-08-08  6832  
c4ed6ebfcb0929 Matthew Wilcox (Oracle  2021-06-29  6833) 		folio->memcg_data = 0;
b4e0b68fbd9d1f Muchun Song             2021-04-29  6834  	}
b4e0b68fbd9d1f Muchun Song             2021-04-29  6835  
b4e0b68fbd9d1f Muchun Song             2021-04-29  6836  	css_put(&memcg->css);
a9d5adeeb4b2c7 Jérôme Glisse           2017-09-08  6837  }
a9d5adeeb4b2c7 Jérôme Glisse           2017-09-08  6838  

---
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