[PATCH v2 0/8] Replace PREEMPT_RT ifdefs with preempt_[dis|en]able_nested().

[PATCH v2 0/8] Replace PREEMPT_RT ifdefs with preempt_[dis|en]able_nested().

[Sebastian Andrzej Siewior]

Folks,

this is v2 of "Replace PREEMPT_RT ifdefs with preempt_[dis|en]able_nested()."
The v1 is at
   https://lore.kernel.org/all/20220817162703.728679-1-bigeasy@linutronix.de/

v1…v2:
   - The SLUB patch got s/use_lockless_fast_path/USE_LOCKLESS_FAST_PATH/ as
     Linus asked for and was handed over to Vlastimil and is sitting in his
     slab tree:
        https://git.kernel.org/vbabka/slab/c/591570a7

   - The compaction patch got an "depends on COMPACTION" so it is not listed
     .config if compaction is disabled (Rasmus Villemoes).

   - The removal of u64_stats_fetch_begin_irq() has been excluded from the
     series and staged for later. Jakub asked for this, so that it can be
     included in later merge window.

   - A patch for flex_proportions has been added which also needs a
     preempt_disable_nested() since the merge window.

Original cover letter:

this is the follow up to the "vfs.git pile 3 - dcache" pull request [0].
It was concluded that the introduction of
	preempt_disable_nested()

in general makes sense and should be used in places where preemption on
!RT is disabled by other means and PREEMPT_RT need to explicitly disable
it.

This series introduces the macro and converts already existing users to
that macro. The u64_stat interface was simplified to make the change
simpler and code easier to follow.

[0] https://lore.kernel.org/all/YurA3aSb4GRr4wlW@ZenIV/

Sebastian

[PATCH v2 1/8] preempt: Provide preempt_[dis|en]able_nested()

[Sebastian Andrzej Siewior]

From: Thomas Gleixner <tglx@linutronix.de>

On PREEMPT_RT enabled kernels, spinlocks and rwlocks are neither disabling
preemption nor interrupts. Though there are a few places which depend on
the implicit preemption/interrupt disable of those locks, e.g. seqcount
write sections, per CPU statistics updates etc.

To avoid sprinkling CONFIG_PREEMPT_RT conditionals all over the place, add
preempt_disable_nested() and preempt_enable_nested() which should be
descriptive enough.

Add a lockdep assertion for the !PREEMPT_RT case to catch callers which
do not have preemption disabled.

Cc: Ben Segall <bsegall@google.com>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/preempt.h | 42 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index b4381f255a5ca..0df425bf9bd75 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -421,4 +421,46 @@ static inline void migrate_enable(void) { }
 
 #endif /* CONFIG_SMP */
 
+/**
+ * preempt_disable_nested - Disable preemption inside a normally preempt disabled section
+ *
+ * Use for code which requires preemption protection inside a critical
+ * section which has preemption disabled implicitly on non-PREEMPT_RT
+ * enabled kernels, by e.g.:
+ *  - holding a spinlock/rwlock
+ *  - soft interrupt context
+ *  - regular interrupt handlers
+ *
+ * On PREEMPT_RT enabled kernels spinlock/rwlock held sections, soft
+ * interrupt context and regular interrupt handlers are preemptible and
+ * only prevent migration. preempt_disable_nested() ensures that preemption
+ * is disabled for cases which require CPU local serialization even on
+ * PREEMPT_RT. For non-PREEMPT_RT kernels this is a NOP.
+ *
+ * The use cases are code sequences which are not serialized by a
+ * particular lock instance, e.g.:
+ *  - seqcount write side critical sections where the seqcount is not
+ *    associated to a particular lock and therefore the automatic
+ *    protection mechanism does not work. This prevents a live lock
+ *    against a preempting high priority reader.
+ *  - RMW per CPU variable updates like vmstat.
+ */
+/* Macro to avoid header recursion hell vs. lockdep */
+#define preempt_disable_nested()				\
+do {								\
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))			\
+		preempt_disable();				\
+	else							\
+		lockdep_assert_preemption_disabled();		\
+} while (0)
+
+/**
+ * preempt_enable_nested - Undo the effect of preempt_disable_nested()
+ */
+static __always_inline void preempt_enable_nested(void)
+{
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable();
+}
+
 #endif /* __LINUX_PREEMPT_H */
-- 
2.37.2

[PATCH v2 2/8] dentry: Use preempt_[dis|en]able_nested()

[Sebastian Andrzej Siewior]

From: Thomas Gleixner <tglx@linutronix.de>

Replace the open coded CONFIG_PREEMPT_RT conditional
preempt_disable/enable() with the new helper.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 fs/dcache.c | 13 ++-----------
 1 file changed, 2 insertions(+), 11 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index bb0c4d0038dbd..2ee8636016ee9 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2597,15 +2597,7 @@ EXPORT_SYMBOL(d_rehash);
 
 static inline unsigned start_dir_add(struct inode *dir)
 {
-	/*
-	 * The caller holds a spinlock (dentry::d_lock). On !PREEMPT_RT
-	 * kernels spin_lock() implicitly disables preemption, but not on
-	 * PREEMPT_RT.  So for RT it has to be done explicitly to protect
-	 * the sequence count write side critical section against a reader
-	 * or another writer preempting, which would result in a live lock.
-	 */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 	for (;;) {
 		unsigned n = dir->i_dir_seq;
 		if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
@@ -2618,8 +2610,7 @@ static inline void end_dir_add(struct inode *dir, unsigned int n,
 			       wait_queue_head_t *d_wait)
 {
 	smp_store_release(&dir->i_dir_seq, n + 2);
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
+	preempt_enable_nested();
 	wake_up_all(d_wait);
 }
 
-- 
2.37.2

[PATCH v2 3/8] mm/vmstat: Use preempt_[dis|en]able_nested()

[Sebastian Andrzej Siewior]

From: Thomas Gleixner <tglx@linutronix.de>

Replace the open coded CONFIG_PREEMPT_RT conditional
preempt_enable/disable() pairs with the new helper functions which hide
the underlying implementation details.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 mm/vmstat.c | 36 ++++++++++++------------------------
 1 file changed, 12 insertions(+), 24 deletions(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 373d2730fcf21..d514fe7f90af0 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -355,8 +355,7 @@ void __mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
 	 * CPU migrations and preemption potentially corrupts a counter so
 	 * disable preemption.
 	 */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 
 	x = delta + __this_cpu_read(*p);
 
@@ -368,8 +367,7 @@ void __mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
 	}
 	__this_cpu_write(*p, x);
 
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
+	preempt_enable_nested();
 }
 EXPORT_SYMBOL(__mod_zone_page_state);
 
@@ -393,8 +391,7 @@ void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
 	}
 
 	/* See __mod_node_page_state */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 
 	x = delta + __this_cpu_read(*p);
 
@@ -406,8 +403,7 @@ void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
 	}
 	__this_cpu_write(*p, x);
 
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
+	preempt_enable_nested();
 }
 EXPORT_SYMBOL(__mod_node_page_state);
 
@@ -441,8 +437,7 @@ void __inc_zone_state(struct zone *zone, enum zone_stat_item item)
 	s8 v, t;
 
 	/* See __mod_node_page_state */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 
 	v = __this_cpu_inc_return(*p);
 	t = __this_cpu_read(pcp->stat_threshold);
@@ -453,8 +448,7 @@ void __inc_zone_state(struct zone *zone, enum zone_stat_item item)
 		__this_cpu_write(*p, -overstep);
 	}
 
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
+	preempt_enable_nested();
 }
 
 void __inc_node_state(struct pglist_data *pgdat, enum node_stat_item item)
@@ -466,8 +460,7 @@ void __inc_node_state(struct pglist_data *pgdat, enum node_stat_item item)
 	VM_WARN_ON_ONCE(vmstat_item_in_bytes(item));
 
 	/* See __mod_node_page_state */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 
 	v = __this_cpu_inc_return(*p);
 	t = __this_cpu_read(pcp->stat_threshold);
@@ -478,8 +471,7 @@ void __inc_node_state(struct pglist_data *pgdat, enum node_stat_item item)
 		__this_cpu_write(*p, -overstep);
 	}
 
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
+	preempt_enable_nested();
 }
 
 void __inc_zone_page_state(struct page *page, enum zone_stat_item item)
@@ -501,8 +493,7 @@ void __dec_zone_state(struct zone *zone, enum zone_stat_item item)
 	s8 v, t;
 
 	/* See __mod_node_page_state */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 
 	v = __this_cpu_dec_return(*p);
 	t = __this_cpu_read(pcp->stat_threshold);
@@ -513,8 +504,7 @@ void __dec_zone_state(struct zone *zone, enum zone_stat_item item)
 		__this_cpu_write(*p, overstep);
 	}
 
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
+	preempt_enable_nested();
 }
 
 void __dec_node_state(struct pglist_data *pgdat, enum node_stat_item item)
@@ -526,8 +516,7 @@ void __dec_node_state(struct pglist_data *pgdat, enum node_stat_item item)
 	VM_WARN_ON_ONCE(vmstat_item_in_bytes(item));
 
 	/* See __mod_node_page_state */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 
 	v = __this_cpu_dec_return(*p);
 	t = __this_cpu_read(pcp->stat_threshold);
@@ -538,8 +527,7 @@ void __dec_node_state(struct pglist_data *pgdat, enum node_stat_item item)
 		__this_cpu_write(*p, overstep);
 	}
 
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
+	preempt_enable_nested();
 }
 
 void __dec_zone_page_state(struct page *page, enum zone_stat_item item)
-- 
2.37.2

[PATCH v2 4/8] mm/debug: Provide VM_WARN_ON_IRQS_ENABLED()

[Sebastian Andrzej Siewior]

From: Thomas Gleixner <tglx@linutronix.de>

Some places in the VM code expect interrupts disabled, which is a valid
expectation on non-PREEMPT_RT kernels, but does not hold on RT kernels in
some places because the RT spinlock substitution does not disable
interrupts.

To avoid sprinkling CONFIG_PREEMPT_RT conditionals into those places,
provide VM_WARN_ON_IRQS_ENABLED() which is only enabled when VM_DEBUG=y and
PREEMPT_RT=n.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/mmdebug.h | 6 ++++++
 lib/Kconfig.debug       | 3 +++
 2 files changed, 9 insertions(+)

diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h
index 15ae78cd28536..b8728d11c9490 100644
--- a/include/linux/mmdebug.h
+++ b/include/linux/mmdebug.h
@@ -94,6 +94,12 @@ void dump_mm(const struct mm_struct *mm);
 #define VM_WARN(cond, format...) BUILD_BUG_ON_INVALID(cond)
 #endif
 
+#ifdef CONFIG_DEBUG_VM_IRQSOFF
+#define VM_WARN_ON_IRQS_ENABLED() WARN_ON_ONCE(!irqs_disabled())
+#else
+#define VM_WARN_ON_IRQS_ENABLED() do { } while (0)
+#endif
+
 #ifdef CONFIG_DEBUG_VIRTUAL
 #define VIRTUAL_BUG_ON(cond) BUG_ON(cond)
 #else
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 072e4b289c13e..c96fc6820544c 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -803,6 +803,9 @@ config ARCH_HAS_DEBUG_VM_PGTABLE
 	  An architecture should select this when it can successfully
 	  build and run DEBUG_VM_PGTABLE.
 
+config DEBUG_VM_IRQSOFF
+	def_bool DEBUG_VM && !PREEMPT_RT
+
 config DEBUG_VM
 	bool "Debug VM"
 	depends on DEBUG_KERNEL
-- 
2.37.2

[PATCH v2 5/8] mm/memcontrol: Replace the PREEMPT_RT conditionals

[Sebastian Andrzej Siewior]

From: Thomas Gleixner <tglx@linutronix.de>

Use VM_WARN_ON_IRQS_ENABLED() and preempt_disable/enable_nested() to
replace the CONFIG_PREEMPT_RT #ifdeffery.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: cgroups@vger.kernel.org
Cc: linux-mm@kvack.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 mm/memcontrol.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b69979c9ced5c..d35b6fa560f0a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -597,25 +597,18 @@ static u64 flush_next_time;
  */
 static void memcg_stats_lock(void)
 {
-#ifdef CONFIG_PREEMPT_RT
-      preempt_disable();
-#else
-      VM_BUG_ON(!irqs_disabled());
-#endif
+	preempt_disable_nested();
+	VM_WARN_ON_IRQS_ENABLED();
 }
 
 static void __memcg_stats_lock(void)
 {
-#ifdef CONFIG_PREEMPT_RT
-      preempt_disable();
-#endif
+	preempt_disable_nested();
 }
 
 static void memcg_stats_unlock(void)
 {
-#ifdef CONFIG_PREEMPT_RT
-      preempt_enable();
-#endif
+	preempt_enable_nested();
 }
 
 static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
@@ -715,7 +708,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	 * interrupt context while other caller need to have disabled interrupt.
 	 */
 	__memcg_stats_lock();
-	if (IS_ENABLED(CONFIG_DEBUG_VM) && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
+	if (IS_ENABLED(CONFIG_DEBUG_VM)) {
 		switch (idx) {
 		case NR_ANON_MAPPED:
 		case NR_FILE_MAPPED:
@@ -725,7 +718,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 			WARN_ON_ONCE(!in_task());
 			break;
 		default:
-			WARN_ON_ONCE(!irqs_disabled());
+			VM_WARN_ON_IRQS_ENABLED();
 		}
 	}
 
-- 
2.37.2

[PATCH v2 5/8] mm/memcontrol: Replace the PREEMPT_RT conditionals

[Sebastian Andrzej Siewior]

From: Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>

Use VM_WARN_ON_IRQS_ENABLED() and preempt_disable/enable_nested() to
replace the CONFIG_PREEMPT_RT #ifdeffery.

Signed-off-by: Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
Cc: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
Cc: Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Roman Gushchin <roman.gushchin-fxUVXftIFDnyG1zEObXtfA@public.gmane.org>
Cc: Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Cc: Muchun Song <songmuchun-EC8Uxl6Npydl57MIdRCFDg@public.gmane.org>
Cc: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Cc: linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
Acked-by: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
Reviewed-by: Muchun Song <songmuchun-EC8Uxl6Npydl57MIdRCFDg@public.gmane.org>
Acked-by: Peter Zijlstra (Intel) <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
---
 mm/memcontrol.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b69979c9ced5c..d35b6fa560f0a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -597,25 +597,18 @@ static u64 flush_next_time;
  */
 static void memcg_stats_lock(void)
 {
-#ifdef CONFIG_PREEMPT_RT
-      preempt_disable();
-#else
-      VM_BUG_ON(!irqs_disabled());
-#endif
+	preempt_disable_nested();
+	VM_WARN_ON_IRQS_ENABLED();
 }
 
 static void __memcg_stats_lock(void)
 {
-#ifdef CONFIG_PREEMPT_RT
-      preempt_disable();
-#endif
+	preempt_disable_nested();
 }
 
 static void memcg_stats_unlock(void)
 {
-#ifdef CONFIG_PREEMPT_RT
-      preempt_enable();
-#endif
+	preempt_enable_nested();
 }
 
 static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
@@ -715,7 +708,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	 * interrupt context while other caller need to have disabled interrupt.
 	 */
 	__memcg_stats_lock();
-	if (IS_ENABLED(CONFIG_DEBUG_VM) && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
+	if (IS_ENABLED(CONFIG_DEBUG_VM)) {
 		switch (idx) {
 		case NR_ANON_MAPPED:
 		case NR_FILE_MAPPED:
@@ -725,7 +718,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 			WARN_ON_ONCE(!in_task());
 			break;
 		default:
-			WARN_ON_ONCE(!irqs_disabled());
+			VM_WARN_ON_IRQS_ENABLED();
 		}
 	}
 
-- 
2.37.2

[PATCH v2 6/8] mm/compaction: Get rid of RT ifdeffery

[Sebastian Andrzej Siewior]

From: Thomas Gleixner <tglx@linutronix.de>

Move the RT dependency for the initial value of
sysctl_compact_unevictable_allowed into Kconfig.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Nick Terrell <terrelln@fb.com>
Cc: linux-mm@kvack.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 mm/Kconfig      | 6 ++++++
 mm/compaction.c | 6 +-----
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index 0331f1461f81c..3897e924e40f2 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -579,6 +579,12 @@ config COMPACTION
 	  it and then we would be really interested to hear about that at
 	  linux-mm@kvack.org.
 
+config COMPACT_UNEVICTABLE_DEFAULT
+	int
+	depends on COMPACTION
+	default 0 if PREEMPT_RT
+	default 1
+
 #
 # support for free page reporting
 config PAGE_REPORTING
diff --git a/mm/compaction.c b/mm/compaction.c
index 640fa76228dd9..10561cb1aaad9 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1727,11 +1727,7 @@ typedef enum {
  * Allow userspace to control policy on scanning the unevictable LRU for
  * compactable pages.
  */
-#ifdef CONFIG_PREEMPT_RT
-int sysctl_compact_unevictable_allowed __read_mostly = 0;
-#else
-int sysctl_compact_unevictable_allowed __read_mostly = 1;
-#endif
+int sysctl_compact_unevictable_allowed __read_mostly = CONFIG_COMPACT_UNEVICTABLE_DEFAULT;
 
 static inline void
 update_fast_start_pfn(struct compact_control *cc, unsigned long pfn)
-- 
2.37.2

[PATCH v2 7/8] flex_proportions: Disable preemption entering the write section.

[Sebastian Andrzej Siewior]

The seqcount fprop_global::sequence is not associated with a lock. The
write section (fprop_new_period()) is invoked from a timer and since the
softirq is preemptible on PREEMPT_RT it is possible to preempt the write
section which is not desited.

Disable preemption around the write section on PREEMPT_RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 lib/flex_proportions.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/flex_proportions.c b/lib/flex_proportions.c
index 05cccbcf1661a..83332fefa6f42 100644
--- a/lib/flex_proportions.c
+++ b/lib/flex_proportions.c
@@ -70,6 +70,7 @@ bool fprop_new_period(struct fprop_global *p, int periods)
 	 */
 	if (events <= 1)
 		return false;
+	preempt_disable_nested();
 	write_seqcount_begin(&p->sequence);
 	if (periods < 64)
 		events -= events >> periods;
@@ -77,6 +78,7 @@ bool fprop_new_period(struct fprop_global *p, int periods)
 	percpu_counter_add(&p->events, -events);
 	p->period += periods;
 	write_seqcount_end(&p->sequence);
+	preempt_enable_nested();
 
 	return true;
 }
-- 
2.37.2

[PATCH v2 8/8] u64_stats: Streamline the implementation

[Sebastian Andrzej Siewior]

From: Thomas Gleixner <tglx@linutronix.de>

The u64 stats code handles 3 different cases:

  - 32bit UP
  - 32bit SMP
  - 64bit

with an unreadable #ifdef maze, which was recently expanded with PREEMPT_RT
conditionals.

Reduce it to two cases (32bit and 64bit) and drop the optimization for
32bit UP as suggested by Linus.

Use the new preempt_disable/enable_nested() helpers to get rid of the
CONFIG_PREEMPT_RT conditionals.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: netdev@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/u64_stats_sync.h | 147 +++++++++++++++------------------
 1 file changed, 65 insertions(+), 82 deletions(-)

diff --git a/include/linux/u64_stats_sync.h b/include/linux/u64_stats_sync.h
index 6ad4e9032d538..46040d66334a8 100644
--- a/include/linux/u64_stats_sync.h
+++ b/include/linux/u64_stats_sync.h
@@ -8,7 +8,7 @@
  *
  * Key points :
  *
- * -  Use a seqcount on 32-bit SMP, only disable preemption for 32-bit UP.
+ * -  Use a seqcount on 32-bit
  * -  The whole thing is a no-op on 64-bit architectures.
  *
  * Usage constraints:
@@ -20,7 +20,8 @@
  *    writer and also spin forever.
  *
  * 3) Write side must use the _irqsave() variant if other writers, or a reader,
- *    can be invoked from an IRQ context.
+ *    can be invoked from an IRQ context. On 64bit systems this variant does not
+ *    disable interrupts.
  *
  * 4) If reader fetches several counters, there is no guarantee the whole values
  *    are consistent w.r.t. each other (remember point #2: seqcounts are not
@@ -29,11 +30,6 @@
  * 5) Readers are allowed to sleep or be preempted/interrupted: they perform
  *    pure reads.
  *
- * 6) Readers must use both u64_stats_fetch_{begin,retry}_irq() if the stats
- *    might be updated from a hardirq or softirq context (remember point #1:
- *    seqcounts are not used for UP kernels). 32-bit UP stat readers could read
- *    corrupted 64-bit values otherwise.
- *
  * Usage :
  *
  * Stats producer (writer) should use following template granted it already got
@@ -66,7 +62,7 @@
 #include <linux/seqlock.h>
 
 struct u64_stats_sync {
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
+#if BITS_PER_LONG == 32
 	seqcount_t	seq;
 #endif
 };
@@ -98,7 +94,22 @@ static inline void u64_stats_inc(u64_stats_t *p)
 	local64_inc(&p->v);
 }
 
-#else
+static inline void u64_stats_init(struct u64_stats_sync *syncp) { }
+static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp) { }
+static inline void __u64_stats_update_end(struct u64_stats_sync *syncp) { }
+static inline unsigned long __u64_stats_irqsave(void) { return 0; }
+static inline void __u64_stats_irqrestore(unsigned long flags) { }
+static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
+{
+	return 0;
+}
+static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
+					   unsigned int start)
+{
+	return false;
+}
+
+#else /* 64 bit */
 
 typedef struct {
 	u64		v;
@@ -123,123 +134,95 @@ static inline void u64_stats_inc(u64_stats_t *p)
 {
 	p->v++;
 }
-#endif
 
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
-#define u64_stats_init(syncp)	seqcount_init(&(syncp)->seq)
-#else
 static inline void u64_stats_init(struct u64_stats_sync *syncp)
 {
+	seqcount_init(&syncp->seq);
 }
-#endif
 
-static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
+static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 	write_seqcount_begin(&syncp->seq);
-#endif
 }
 
-static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
+static inline void __u64_stats_update_end(struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
 	write_seqcount_end(&syncp->seq);
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
-#endif
+	preempt_enable_nested();
 }
 
-static inline unsigned long
-u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
+static inline unsigned long __u64_stats_irqsave(void)
 {
-	unsigned long flags = 0;
+	unsigned long flags;
 
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
-	else
-		local_irq_save(flags);
-	write_seqcount_begin(&syncp->seq);
-#endif
+	local_irq_save(flags);
 	return flags;
 }
 
-static inline void
-u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
-				unsigned long flags)
+static inline void __u64_stats_irqrestore(unsigned long flags)
 {
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
-	write_seqcount_end(&syncp->seq);
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
-	else
-		local_irq_restore(flags);
-#endif
+	local_irq_restore(flags);
 }
 
 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
 	return read_seqcount_begin(&syncp->seq);
-#else
-	return 0;
-#endif
+}
+
+static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
+					   unsigned int start)
+{
+	return read_seqcount_retry(&syncp->seq, start);
+}
+#endif /* !64 bit */
+
+static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
+{
+	__u64_stats_update_begin(syncp);
+}
+
+static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
+{
+	__u64_stats_update_end(syncp);
+}
+
+static inline unsigned long u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
+{
+	unsigned long flags = __u64_stats_irqsave();
+
+	__u64_stats_update_begin(syncp);
+	return flags;
+}
+
+static inline void u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
+						   unsigned long flags)
+{
+	__u64_stats_update_end(syncp);
+	__u64_stats_irqrestore(flags);
 }
 
 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG == 32 && (!defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT))
-	preempt_disable();
-#endif
 	return __u64_stats_fetch_begin(syncp);
 }
 
-static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
-					 unsigned int start)
-{
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
-	return read_seqcount_retry(&syncp->seq, start);
-#else
-	return false;
-#endif
-}
-
 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 					 unsigned int start)
 {
-#if BITS_PER_LONG == 32 && (!defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT))
-	preempt_enable();
-#endif
 	return __u64_stats_fetch_retry(syncp, start);
 }
 
-/*
- * In case irq handlers can update u64 counters, readers can use following helpers
- * - SMP 32bit arches use seqcount protection, irq safe.
- * - UP 32bit must disable irqs.
- * - 64bit have no problem atomically reading u64 values, irq safe.
- */
+/* Obsolete interfaces */
 static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT_RT)
-	preempt_disable();
-#elif BITS_PER_LONG == 32 && !defined(CONFIG_SMP)
-	local_irq_disable();
-#endif
-	return __u64_stats_fetch_begin(syncp);
+	return u64_stats_fetch_begin(syncp);
 }
 
 static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
 					     unsigned int start)
 {
-#if BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT_RT)
-	preempt_enable();
-#elif BITS_PER_LONG == 32 && !defined(CONFIG_SMP)
-	local_irq_enable();
-#endif
-	return __u64_stats_fetch_retry(syncp, start);
+	return u64_stats_fetch_retry(syncp, start);
 }
 
 #endif /* _LINUX_U64_STATS_SYNC_H */
-- 
2.37.2

Re: [PATCH v2 2/8] dentry: Use preempt_[dis|en]able_nested()

[Christian Brauner]

On Thu, Aug 25, 2022 at 06:41:25PM +0200, Sebastian Andrzej Siewior wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> Replace the open coded CONFIG_PREEMPT_RT conditional
> preempt_disable/enable() with the new helper.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: linux-fsdevel@vger.kernel.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---

Acked-by: Christian Brauner (Microsoft) <brauner@kernel.org>

Re: [PATCH v2 3/8] mm/vmstat: Use preempt_[dis|en]able_nested()

[Michal Hocko]

On Thu 25-08-22 18:41:26, Sebastian Andrzej Siewior wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> Replace the open coded CONFIG_PREEMPT_RT conditional
> preempt_enable/disable() pairs with the new helper functions which hide
> the underlying implementation details.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: linux-mm@kvack.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Acked-by: Michal Hocko <mhocko@suse.com>
Thanks!

> ---
>  mm/vmstat.c | 36 ++++++++++++------------------------
>  1 file changed, 12 insertions(+), 24 deletions(-)
> 
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 373d2730fcf21..d514fe7f90af0 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -355,8 +355,7 @@ void __mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
>  	 * CPU migrations and preemption potentially corrupts a counter so
>  	 * disable preemption.
>  	 */
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		preempt_disable();
> +	preempt_disable_nested();
>  
>  	x = delta + __this_cpu_read(*p);
>  
> @@ -368,8 +367,7 @@ void __mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
>  	}
>  	__this_cpu_write(*p, x);
>  
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		preempt_enable();
> +	preempt_enable_nested();
>  }
>  EXPORT_SYMBOL(__mod_zone_page_state);
>  
> @@ -393,8 +391,7 @@ void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
>  	}
>  
>  	/* See __mod_node_page_state */
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		preempt_disable();
> +	preempt_disable_nested();
>  
>  	x = delta + __this_cpu_read(*p);
>  
> @@ -406,8 +403,7 @@ void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
>  	}
>  	__this_cpu_write(*p, x);
>  
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		preempt_enable();
> +	preempt_enable_nested();
>  }
>  EXPORT_SYMBOL(__mod_node_page_state);
>  
> @@ -441,8 +437,7 @@ void __inc_zone_state(struct zone *zone, enum zone_stat_item item)
>  	s8 v, t;
>  
>  	/* See __mod_node_page_state */
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		preempt_disable();
> +	preempt_disable_nested();
>  
>  	v = __this_cpu_inc_return(*p);
>  	t = __this_cpu_read(pcp->stat_threshold);
> @@ -453,8 +448,7 @@ void __inc_zone_state(struct zone *zone, enum zone_stat_item item)
>  		__this_cpu_write(*p, -overstep);
>  	}
>  
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		preempt_enable();
> +	preempt_enable_nested();
>  }
>  
>  void __inc_node_state(struct pglist_data *pgdat, enum node_stat_item item)
> @@ -466,8 +460,7 @@ void __inc_node_state(struct pglist_data *pgdat, enum node_stat_item item)
>  	VM_WARN_ON_ONCE(vmstat_item_in_bytes(item));
>  
>  	/* See __mod_node_page_state */
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		preempt_disable();
> +	preempt_disable_nested();
>  
>  	v = __this_cpu_inc_return(*p);
>  	t = __this_cpu_read(pcp->stat_threshold);
> @@ -478,8 +471,7 @@ void __inc_node_state(struct pglist_data *pgdat, enum node_stat_item item)
>  		__this_cpu_write(*p, -overstep);
>  	}
>  
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		preempt_enable();
> +	preempt_enable_nested();
>  }
>  
>  void __inc_zone_page_state(struct page *page, enum zone_stat_item item)
> @@ -501,8 +493,7 @@ void __dec_zone_state(struct zone *zone, enum zone_stat_item item)
>  	s8 v, t;
>  
>  	/* See __mod_node_page_state */
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		preempt_disable();
> +	preempt_disable_nested();
>  
>  	v = __this_cpu_dec_return(*p);
>  	t = __this_cpu_read(pcp->stat_threshold);
> @@ -513,8 +504,7 @@ void __dec_zone_state(struct zone *zone, enum zone_stat_item item)
>  		__this_cpu_write(*p, overstep);
>  	}
>  
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		preempt_enable();
> +	preempt_enable_nested();
>  }
>  
>  void __dec_node_state(struct pglist_data *pgdat, enum node_stat_item item)
> @@ -526,8 +516,7 @@ void __dec_node_state(struct pglist_data *pgdat, enum node_stat_item item)
>  	VM_WARN_ON_ONCE(vmstat_item_in_bytes(item));
>  
>  	/* See __mod_node_page_state */
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		preempt_disable();
> +	preempt_disable_nested();
>  
>  	v = __this_cpu_dec_return(*p);
>  	t = __this_cpu_read(pcp->stat_threshold);
> @@ -538,8 +527,7 @@ void __dec_node_state(struct pglist_data *pgdat, enum node_stat_item item)
>  		__this_cpu_write(*p, overstep);
>  	}
>  
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		preempt_enable();
> +	preempt_enable_nested();
>  }
>  
>  void __dec_zone_page_state(struct page *page, enum zone_stat_item item)
> -- 
> 2.37.2

-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2 4/8] mm/debug: Provide VM_WARN_ON_IRQS_ENABLED()

[Michal Hocko]

On Thu 25-08-22 18:41:27, Sebastian Andrzej Siewior wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> Some places in the VM code expect interrupts disabled, which is a valid
> expectation on non-PREEMPT_RT kernels, but does not hold on RT kernels in
> some places because the RT spinlock substitution does not disable
> interrupts.
> 
> To avoid sprinkling CONFIG_PREEMPT_RT conditionals into those places,
> provide VM_WARN_ON_IRQS_ENABLED() which is only enabled when VM_DEBUG=y and
> PREEMPT_RT=n.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: linux-mm@kvack.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Acked-by: Michal Hocko <mhocko@suse.com>
Thanks!

> ---
>  include/linux/mmdebug.h | 6 ++++++
>  lib/Kconfig.debug       | 3 +++
>  2 files changed, 9 insertions(+)
> 
> diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h
> index 15ae78cd28536..b8728d11c9490 100644
> --- a/include/linux/mmdebug.h
> +++ b/include/linux/mmdebug.h
> @@ -94,6 +94,12 @@ void dump_mm(const struct mm_struct *mm);
>  #define VM_WARN(cond, format...) BUILD_BUG_ON_INVALID(cond)
>  #endif
>  
> +#ifdef CONFIG_DEBUG_VM_IRQSOFF
> +#define VM_WARN_ON_IRQS_ENABLED() WARN_ON_ONCE(!irqs_disabled())
> +#else
> +#define VM_WARN_ON_IRQS_ENABLED() do { } while (0)
> +#endif
> +
>  #ifdef CONFIG_DEBUG_VIRTUAL
>  #define VIRTUAL_BUG_ON(cond) BUG_ON(cond)
>  #else
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 072e4b289c13e..c96fc6820544c 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -803,6 +803,9 @@ config ARCH_HAS_DEBUG_VM_PGTABLE
>  	  An architecture should select this when it can successfully
>  	  build and run DEBUG_VM_PGTABLE.
>  
> +config DEBUG_VM_IRQSOFF
> +	def_bool DEBUG_VM && !PREEMPT_RT
> +
>  config DEBUG_VM
>  	bool "Debug VM"
>  	depends on DEBUG_KERNEL
> -- 
> 2.37.2

-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2 5/8] mm/memcontrol: Replace the PREEMPT_RT conditionals

[Michal Hocko]

On Thu 25-08-22 18:41:28, Sebastian Andrzej Siewior wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> Use VM_WARN_ON_IRQS_ENABLED() and preempt_disable/enable_nested() to
> replace the CONFIG_PREEMPT_RT #ifdeffery.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Roman Gushchin <roman.gushchin@linux.dev>
> Cc: Shakeel Butt <shakeelb@google.com>
> Cc: Muchun Song <songmuchun@bytedance.com>
> Cc: cgroups@vger.kernel.org
> Cc: linux-mm@kvack.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Acked-by: Michal Hocko <mhocko@suse.com>
Thanks!

> ---
>  mm/memcontrol.c | 19 ++++++-------------
>  1 file changed, 6 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b69979c9ced5c..d35b6fa560f0a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -597,25 +597,18 @@ static u64 flush_next_time;
>   */
>  static void memcg_stats_lock(void)
>  {
> -#ifdef CONFIG_PREEMPT_RT
> -      preempt_disable();
> -#else
> -      VM_BUG_ON(!irqs_disabled());
> -#endif
> +	preempt_disable_nested();
> +	VM_WARN_ON_IRQS_ENABLED();
>  }
>  
>  static void __memcg_stats_lock(void)
>  {
> -#ifdef CONFIG_PREEMPT_RT
> -      preempt_disable();
> -#endif
> +	preempt_disable_nested();
>  }
>  
>  static void memcg_stats_unlock(void)
>  {
> -#ifdef CONFIG_PREEMPT_RT
> -      preempt_enable();
> -#endif
> +	preempt_enable_nested();
>  }
>  
>  static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
> @@ -715,7 +708,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
>  	 * interrupt context while other caller need to have disabled interrupt.
>  	 */
>  	__memcg_stats_lock();
> -	if (IS_ENABLED(CONFIG_DEBUG_VM) && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
> +	if (IS_ENABLED(CONFIG_DEBUG_VM)) {
>  		switch (idx) {
>  		case NR_ANON_MAPPED:
>  		case NR_FILE_MAPPED:
> @@ -725,7 +718,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
>  			WARN_ON_ONCE(!in_task());
>  			break;
>  		default:
> -			WARN_ON_ONCE(!irqs_disabled());
> +			VM_WARN_ON_IRQS_ENABLED();
>  		}
>  	}
>  
> -- 
> 2.37.2

-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2 5/8] mm/memcontrol: Replace the PREEMPT_RT conditionals

[Michal Hocko]

On Thu 25-08-22 18:41:28, Sebastian Andrzej Siewior wrote:
> From: Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
> 
> Use VM_WARN_ON_IRQS_ENABLED() and preempt_disable/enable_nested() to
> replace the CONFIG_PREEMPT_RT #ifdeffery.
> 
> Signed-off-by: Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
> Cc: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
> Cc: Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Cc: Roman Gushchin <roman.gushchin-fxUVXftIFDnyG1zEObXtfA@public.gmane.org>
> Cc: Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> Cc: Muchun Song <songmuchun-EC8Uxl6Npydl57MIdRCFDg@public.gmane.org>
> Cc: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> Cc: linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
> Acked-by: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
> Reviewed-by: Muchun Song <songmuchun-EC8Uxl6Npydl57MIdRCFDg@public.gmane.org>
> Acked-by: Peter Zijlstra (Intel) <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>

Acked-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
Thanks!

> ---
>  mm/memcontrol.c | 19 ++++++-------------
>  1 file changed, 6 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b69979c9ced5c..d35b6fa560f0a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -597,25 +597,18 @@ static u64 flush_next_time;
>   */
>  static void memcg_stats_lock(void)
>  {
> -#ifdef CONFIG_PREEMPT_RT
> -      preempt_disable();
> -#else
> -      VM_BUG_ON(!irqs_disabled());
> -#endif
> +	preempt_disable_nested();
> +	VM_WARN_ON_IRQS_ENABLED();
>  }
>  
>  static void __memcg_stats_lock(void)
>  {
> -#ifdef CONFIG_PREEMPT_RT
> -      preempt_disable();
> -#endif
> +	preempt_disable_nested();
>  }
>  
>  static void memcg_stats_unlock(void)
>  {
> -#ifdef CONFIG_PREEMPT_RT
> -      preempt_enable();
> -#endif
> +	preempt_enable_nested();
>  }
>  
>  static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
> @@ -715,7 +708,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
>  	 * interrupt context while other caller need to have disabled interrupt.
>  	 */
>  	__memcg_stats_lock();
> -	if (IS_ENABLED(CONFIG_DEBUG_VM) && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
> +	if (IS_ENABLED(CONFIG_DEBUG_VM)) {
>  		switch (idx) {
>  		case NR_ANON_MAPPED:
>  		case NR_FILE_MAPPED:
> @@ -725,7 +718,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
>  			WARN_ON_ONCE(!in_task());
>  			break;
>  		default:
> -			WARN_ON_ONCE(!irqs_disabled());
> +			VM_WARN_ON_IRQS_ENABLED();
>  		}
>  	}
>  
> -- 
> 2.37.2

-- 
Michal Hocko
SUSE Labs

[tip: sched/rt] u64_stats: Streamline the implementation

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the sched/rt branch of tip:

Commit-ID:     44b0c2957adc62b86fcd51adeaf8e993171bc319
Gitweb:        https://git.kernel.org/tip/44b0c2957adc62b86fcd51adeaf8e993171bc319
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Thu, 25 Aug 2022 18:41:31 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Mon, 19 Sep 2022 14:35:08 +02:00

u64_stats: Streamline the implementation

The u64 stats code handles 3 different cases:

  - 32bit UP
  - 32bit SMP
  - 64bit

with an unreadable #ifdef maze, which was recently expanded with PREEMPT_RT
conditionals.

Reduce it to two cases (32bit and 64bit) and drop the optimization for
32bit UP as suggested by Linus.

Use the new preempt_disable/enable_nested() helpers to get rid of the
CONFIG_PREEMPT_RT conditionals.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220825164131.402717-9-bigeasy@linutronix.de

---
 include/linux/u64_stats_sync.h | 145 ++++++++++++++------------------
 1 file changed, 64 insertions(+), 81 deletions(-)

diff --git a/include/linux/u64_stats_sync.h b/include/linux/u64_stats_sync.h
index 6ad4e90..46040d6 100644
--- a/include/linux/u64_stats_sync.h
+++ b/include/linux/u64_stats_sync.h
@@ -8,7 +8,7 @@
  *
  * Key points :
  *
- * -  Use a seqcount on 32-bit SMP, only disable preemption for 32-bit UP.
+ * -  Use a seqcount on 32-bit
  * -  The whole thing is a no-op on 64-bit architectures.
  *
  * Usage constraints:
@@ -20,7 +20,8 @@
  *    writer and also spin forever.
  *
  * 3) Write side must use the _irqsave() variant if other writers, or a reader,
- *    can be invoked from an IRQ context.
+ *    can be invoked from an IRQ context. On 64bit systems this variant does not
+ *    disable interrupts.
  *
  * 4) If reader fetches several counters, there is no guarantee the whole values
  *    are consistent w.r.t. each other (remember point #2: seqcounts are not
@@ -29,11 +30,6 @@
  * 5) Readers are allowed to sleep or be preempted/interrupted: they perform
  *    pure reads.
  *
- * 6) Readers must use both u64_stats_fetch_{begin,retry}_irq() if the stats
- *    might be updated from a hardirq or softirq context (remember point #1:
- *    seqcounts are not used for UP kernels). 32-bit UP stat readers could read
- *    corrupted 64-bit values otherwise.
- *
  * Usage :
  *
  * Stats producer (writer) should use following template granted it already got
@@ -66,7 +62,7 @@
 #include <linux/seqlock.h>
 
 struct u64_stats_sync {
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
+#if BITS_PER_LONG == 32
 	seqcount_t	seq;
 #endif
 };
@@ -98,7 +94,22 @@ static inline void u64_stats_inc(u64_stats_t *p)
 	local64_inc(&p->v);
 }
 
-#else
+static inline void u64_stats_init(struct u64_stats_sync *syncp) { }
+static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp) { }
+static inline void __u64_stats_update_end(struct u64_stats_sync *syncp) { }
+static inline unsigned long __u64_stats_irqsave(void) { return 0; }
+static inline void __u64_stats_irqrestore(unsigned long flags) { }
+static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
+{
+	return 0;
+}
+static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
+					   unsigned int start)
+{
+	return false;
+}
+
+#else /* 64 bit */
 
 typedef struct {
 	u64		v;
@@ -123,123 +134,95 @@ static inline void u64_stats_inc(u64_stats_t *p)
 {
 	p->v++;
 }
-#endif
 
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
-#define u64_stats_init(syncp)	seqcount_init(&(syncp)->seq)
-#else
 static inline void u64_stats_init(struct u64_stats_sync *syncp)
 {
+	seqcount_init(&syncp->seq);
 }
-#endif
 
-static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
+static inline void __u64_stats_update_begin(struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 	write_seqcount_begin(&syncp->seq);
-#endif
 }
 
-static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
+static inline void __u64_stats_update_end(struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
 	write_seqcount_end(&syncp->seq);
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
-#endif
+	preempt_enable_nested();
 }
 
-static inline unsigned long
-u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
+static inline unsigned long __u64_stats_irqsave(void)
 {
-	unsigned long flags = 0;
+	unsigned long flags;
 
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
-	else
-		local_irq_save(flags);
-	write_seqcount_begin(&syncp->seq);
-#endif
+	local_irq_save(flags);
 	return flags;
 }
 
-static inline void
-u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
-				unsigned long flags)
+static inline void __u64_stats_irqrestore(unsigned long flags)
 {
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
-	write_seqcount_end(&syncp->seq);
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
-	else
-		local_irq_restore(flags);
-#endif
+	local_irq_restore(flags);
 }
 
 static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
 	return read_seqcount_begin(&syncp->seq);
-#else
-	return 0;
-#endif
 }
 
-static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
+static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
+					   unsigned int start)
 {
-#if BITS_PER_LONG == 32 && (!defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT))
-	preempt_disable();
-#endif
-	return __u64_stats_fetch_begin(syncp);
+	return read_seqcount_retry(&syncp->seq, start);
 }
+#endif /* !64 bit */
 
-static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
-					 unsigned int start)
+static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
-	return read_seqcount_retry(&syncp->seq, start);
-#else
-	return false;
-#endif
+	__u64_stats_update_begin(syncp);
+}
+
+static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
+{
+	__u64_stats_update_end(syncp);
+}
+
+static inline unsigned long u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
+{
+	unsigned long flags = __u64_stats_irqsave();
+
+	__u64_stats_update_begin(syncp);
+	return flags;
+}
+
+static inline void u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
+						   unsigned long flags)
+{
+	__u64_stats_update_end(syncp);
+	__u64_stats_irqrestore(flags);
+}
+
+static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
+{
+	return __u64_stats_fetch_begin(syncp);
 }
 
 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 					 unsigned int start)
 {
-#if BITS_PER_LONG == 32 && (!defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT))
-	preempt_enable();
-#endif
 	return __u64_stats_fetch_retry(syncp, start);
 }
 
-/*
- * In case irq handlers can update u64 counters, readers can use following helpers
- * - SMP 32bit arches use seqcount protection, irq safe.
- * - UP 32bit must disable irqs.
- * - 64bit have no problem atomically reading u64 values, irq safe.
- */
+/* Obsolete interfaces */
 static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT_RT)
-	preempt_disable();
-#elif BITS_PER_LONG == 32 && !defined(CONFIG_SMP)
-	local_irq_disable();
-#endif
-	return __u64_stats_fetch_begin(syncp);
+	return u64_stats_fetch_begin(syncp);
 }
 
 static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
 					     unsigned int start)
 {
-#if BITS_PER_LONG == 32 && defined(CONFIG_PREEMPT_RT)
-	preempt_enable();
-#elif BITS_PER_LONG == 32 && !defined(CONFIG_SMP)
-	local_irq_enable();
-#endif
-	return __u64_stats_fetch_retry(syncp, start);
+	return u64_stats_fetch_retry(syncp, start);
 }
 
 #endif /* _LINUX_U64_STATS_SYNC_H */

[tip: sched/rt] flex_proportions: Disable preemption entering the write section.

[tip-bot2 for Sebastian Andrzej Siewior]

The following commit has been merged into the sched/rt branch of tip:

Commit-ID:     9458e0a78c45bc6537ce11eb9f03489eab92f9c2
Gitweb:        https://git.kernel.org/tip/9458e0a78c45bc6537ce11eb9f03489eab92f9c2
Author:        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
AuthorDate:    Thu, 25 Aug 2022 18:41:30 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Mon, 19 Sep 2022 14:35:08 +02:00

flex_proportions: Disable preemption entering the write section.

The seqcount fprop_global::sequence is not associated with a lock. The
write section (fprop_new_period()) is invoked from a timer and since the
softirq is preemptible on PREEMPT_RT it is possible to preempt the write
section which is not desited.

Disable preemption around the write section on PREEMPT_RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20220825164131.402717-8-bigeasy@linutronix.de

---
 lib/flex_proportions.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/flex_proportions.c b/lib/flex_proportions.c
index 05cccbc..83332fe 100644
--- a/lib/flex_proportions.c
+++ b/lib/flex_proportions.c
@@ -70,6 +70,7 @@ bool fprop_new_period(struct fprop_global *p, int periods)
 	 */
 	if (events <= 1)
 		return false;
+	preempt_disable_nested();
 	write_seqcount_begin(&p->sequence);
 	if (periods < 64)
 		events -= events >> periods;
@@ -77,6 +78,7 @@ bool fprop_new_period(struct fprop_global *p, int periods)
 	percpu_counter_add(&p->events, -events);
 	p->period += periods;
 	write_seqcount_end(&p->sequence);
+	preempt_enable_nested();
 
 	return true;
 }

[tip: sched/rt] mm/compaction: Get rid of RT ifdeffery

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the sched/rt branch of tip:

Commit-ID:     c7e0b3d088717d148707cd6fcb12f97c6fd961c1
Gitweb:        https://git.kernel.org/tip/c7e0b3d088717d148707cd6fcb12f97c6fd961c1
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Thu, 25 Aug 2022 18:41:29 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Mon, 19 Sep 2022 14:35:08 +02:00

mm/compaction: Get rid of RT ifdeffery

Move the RT dependency for the initial value of
sysctl_compact_unevictable_allowed into Kconfig.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220825164131.402717-7-bigeasy@linutronix.de

---
 mm/Kconfig      | 6 ++++++
 mm/compaction.c | 6 +-----
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index 0331f14..3897e92 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -579,6 +579,12 @@ config COMPACTION
 	  it and then we would be really interested to hear about that at
 	  linux-mm@kvack.org.
 
+config COMPACT_UNEVICTABLE_DEFAULT
+	int
+	depends on COMPACTION
+	default 0 if PREEMPT_RT
+	default 1
+
 #
 # support for free page reporting
 config PAGE_REPORTING
diff --git a/mm/compaction.c b/mm/compaction.c
index 640fa76..10561cb 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1727,11 +1727,7 @@ typedef enum {
  * Allow userspace to control policy on scanning the unevictable LRU for
  * compactable pages.
  */
-#ifdef CONFIG_PREEMPT_RT
-int sysctl_compact_unevictable_allowed __read_mostly = 0;
-#else
-int sysctl_compact_unevictable_allowed __read_mostly = 1;
-#endif
+int sysctl_compact_unevictable_allowed __read_mostly = CONFIG_COMPACT_UNEVICTABLE_DEFAULT;
 
 static inline void
 update_fast_start_pfn(struct compact_control *cc, unsigned long pfn)

[tip: sched/rt] mm/memcontrol: Replace the PREEMPT_RT conditionals

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the sched/rt branch of tip:

Commit-ID:     e575d401583273a7ac5dfb27520e41c821e81816
Gitweb:        https://git.kernel.org/tip/e575d401583273a7ac5dfb27520e41c821e81816
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Thu, 25 Aug 2022 18:41:28 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Mon, 19 Sep 2022 14:35:08 +02:00

mm/memcontrol: Replace the PREEMPT_RT conditionals

Use VM_WARN_ON_IRQS_ENABLED() and preempt_disable/enable_nested() to
replace the CONFIG_PREEMPT_RT #ifdeffery.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Link: https://lore.kernel.org/r/20220825164131.402717-6-bigeasy@linutronix.de

---
 mm/memcontrol.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b69979c..d35b6fa 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -597,25 +597,18 @@ static u64 flush_next_time;
  */
 static void memcg_stats_lock(void)
 {
-#ifdef CONFIG_PREEMPT_RT
-      preempt_disable();
-#else
-      VM_BUG_ON(!irqs_disabled());
-#endif
+	preempt_disable_nested();
+	VM_WARN_ON_IRQS_ENABLED();
 }
 
 static void __memcg_stats_lock(void)
 {
-#ifdef CONFIG_PREEMPT_RT
-      preempt_disable();
-#endif
+	preempt_disable_nested();
 }
 
 static void memcg_stats_unlock(void)
 {
-#ifdef CONFIG_PREEMPT_RT
-      preempt_enable();
-#endif
+	preempt_enable_nested();
 }
 
 static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
@@ -715,7 +708,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 	 * interrupt context while other caller need to have disabled interrupt.
 	 */
 	__memcg_stats_lock();
-	if (IS_ENABLED(CONFIG_DEBUG_VM) && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
+	if (IS_ENABLED(CONFIG_DEBUG_VM)) {
 		switch (idx) {
 		case NR_ANON_MAPPED:
 		case NR_FILE_MAPPED:
@@ -725,7 +718,7 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx,
 			WARN_ON_ONCE(!in_task());
 			break;
 		default:
-			WARN_ON_ONCE(!irqs_disabled());
+			VM_WARN_ON_IRQS_ENABLED();
 		}
 	}
 

[tip: sched/rt] mm/debug: Provide VM_WARN_ON_IRQS_ENABLED()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the sched/rt branch of tip:

Commit-ID:     a738e9bad6030d4fd33bfd7db3399a250b7e94d8
Gitweb:        https://git.kernel.org/tip/a738e9bad6030d4fd33bfd7db3399a250b7e94d8
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Thu, 25 Aug 2022 18:41:27 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Mon, 19 Sep 2022 14:35:08 +02:00

mm/debug: Provide VM_WARN_ON_IRQS_ENABLED()

Some places in the VM code expect interrupts disabled, which is a valid
expectation on non-PREEMPT_RT kernels, but does not hold on RT kernels in
some places because the RT spinlock substitution does not disable
interrupts.

To avoid sprinkling CONFIG_PREEMPT_RT conditionals into those places,
provide VM_WARN_ON_IRQS_ENABLED() which is only enabled when VM_DEBUG=y and
PREEMPT_RT=n.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Link: https://lore.kernel.org/r/20220825164131.402717-5-bigeasy@linutronix.de

---
 include/linux/mmdebug.h | 6 ++++++
 lib/Kconfig.debug       | 3 +++
 2 files changed, 9 insertions(+)

diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h
index 15ae78c..b8728d1 100644
--- a/include/linux/mmdebug.h
+++ b/include/linux/mmdebug.h
@@ -94,6 +94,12 @@ void dump_mm(const struct mm_struct *mm);
 #define VM_WARN(cond, format...) BUILD_BUG_ON_INVALID(cond)
 #endif
 
+#ifdef CONFIG_DEBUG_VM_IRQSOFF
+#define VM_WARN_ON_IRQS_ENABLED() WARN_ON_ONCE(!irqs_disabled())
+#else
+#define VM_WARN_ON_IRQS_ENABLED() do { } while (0)
+#endif
+
 #ifdef CONFIG_DEBUG_VIRTUAL
 #define VIRTUAL_BUG_ON(cond) BUG_ON(cond)
 #else
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index bcbe60d..cdb4b27 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -803,6 +803,9 @@ config ARCH_HAS_DEBUG_VM_PGTABLE
 	  An architecture should select this when it can successfully
 	  build and run DEBUG_VM_PGTABLE.
 
+config DEBUG_VM_IRQSOFF
+	def_bool DEBUG_VM && !PREEMPT_RT
+
 config DEBUG_VM
 	bool "Debug VM"
 	depends on DEBUG_KERNEL

[tip: sched/rt] mm/vmstat: Use preempt_[dis|en]able_nested()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the sched/rt branch of tip:

Commit-ID:     7a025e91abd23effe869a05d037b26770ffa0309
Gitweb:        https://git.kernel.org/tip/7a025e91abd23effe869a05d037b26770ffa0309
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Thu, 25 Aug 2022 18:41:26 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Mon, 19 Sep 2022 14:35:08 +02:00

mm/vmstat: Use preempt_[dis|en]able_nested()

Replace the open coded CONFIG_PREEMPT_RT conditional
preempt_enable/disable() pairs with the new helper functions which hide
the underlying implementation details.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Link: https://lore.kernel.org/r/20220825164131.402717-4-bigeasy@linutronix.de

---
 mm/vmstat.c | 36 ++++++++++++------------------------
 1 file changed, 12 insertions(+), 24 deletions(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 90af9a8..7a2d73f 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -355,8 +355,7 @@ void __mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
 	 * CPU migrations and preemption potentially corrupts a counter so
 	 * disable preemption.
 	 */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 
 	x = delta + __this_cpu_read(*p);
 
@@ -368,8 +367,7 @@ void __mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
 	}
 	__this_cpu_write(*p, x);
 
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
+	preempt_enable_nested();
 }
 EXPORT_SYMBOL(__mod_zone_page_state);
 
@@ -393,8 +391,7 @@ void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
 	}
 
 	/* See __mod_node_page_state */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 
 	x = delta + __this_cpu_read(*p);
 
@@ -406,8 +403,7 @@ void __mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item item,
 	}
 	__this_cpu_write(*p, x);
 
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
+	preempt_enable_nested();
 }
 EXPORT_SYMBOL(__mod_node_page_state);
 
@@ -441,8 +437,7 @@ void __inc_zone_state(struct zone *zone, enum zone_stat_item item)
 	s8 v, t;
 
 	/* See __mod_node_page_state */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 
 	v = __this_cpu_inc_return(*p);
 	t = __this_cpu_read(pcp->stat_threshold);
@@ -453,8 +448,7 @@ void __inc_zone_state(struct zone *zone, enum zone_stat_item item)
 		__this_cpu_write(*p, -overstep);
 	}
 
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
+	preempt_enable_nested();
 }
 
 void __inc_node_state(struct pglist_data *pgdat, enum node_stat_item item)
@@ -466,8 +460,7 @@ void __inc_node_state(struct pglist_data *pgdat, enum node_stat_item item)
 	VM_WARN_ON_ONCE(vmstat_item_in_bytes(item));
 
 	/* See __mod_node_page_state */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 
 	v = __this_cpu_inc_return(*p);
 	t = __this_cpu_read(pcp->stat_threshold);
@@ -478,8 +471,7 @@ void __inc_node_state(struct pglist_data *pgdat, enum node_stat_item item)
 		__this_cpu_write(*p, -overstep);
 	}
 
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
+	preempt_enable_nested();
 }
 
 void __inc_zone_page_state(struct page *page, enum zone_stat_item item)
@@ -501,8 +493,7 @@ void __dec_zone_state(struct zone *zone, enum zone_stat_item item)
 	s8 v, t;
 
 	/* See __mod_node_page_state */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 
 	v = __this_cpu_dec_return(*p);
 	t = __this_cpu_read(pcp->stat_threshold);
@@ -513,8 +504,7 @@ void __dec_zone_state(struct zone *zone, enum zone_stat_item item)
 		__this_cpu_write(*p, overstep);
 	}
 
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
+	preempt_enable_nested();
 }
 
 void __dec_node_state(struct pglist_data *pgdat, enum node_stat_item item)
@@ -526,8 +516,7 @@ void __dec_node_state(struct pglist_data *pgdat, enum node_stat_item item)
 	VM_WARN_ON_ONCE(vmstat_item_in_bytes(item));
 
 	/* See __mod_node_page_state */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 
 	v = __this_cpu_dec_return(*p);
 	t = __this_cpu_read(pcp->stat_threshold);
@@ -538,8 +527,7 @@ void __dec_node_state(struct pglist_data *pgdat, enum node_stat_item item)
 		__this_cpu_write(*p, overstep);
 	}
 
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
+	preempt_enable_nested();
 }
 
 void __dec_zone_page_state(struct page *page, enum zone_stat_item item)

[tip: sched/rt] dentry: Use preempt_[dis|en]able_nested()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the sched/rt branch of tip:

Commit-ID:     93f6d4e1893657f07ba3c9e2bfa74b355a0b32f9
Gitweb:        https://git.kernel.org/tip/93f6d4e1893657f07ba3c9e2bfa74b355a0b32f9
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Thu, 25 Aug 2022 18:41:25 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Mon, 19 Sep 2022 14:35:07 +02:00

dentry: Use preempt_[dis|en]able_nested()

Replace the open coded CONFIG_PREEMPT_RT conditional
preempt_disable/enable() with the new helper.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Link: https://lore.kernel.org/r/20220825164131.402717-3-bigeasy@linutronix.de

---
 fs/dcache.c | 13 ++-----------
 1 file changed, 2 insertions(+), 11 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index bb0c4d0..2ee8636 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2597,15 +2597,7 @@ EXPORT_SYMBOL(d_rehash);
 
 static inline unsigned start_dir_add(struct inode *dir)
 {
-	/*
-	 * The caller holds a spinlock (dentry::d_lock). On !PREEMPT_RT
-	 * kernels spin_lock() implicitly disables preemption, but not on
-	 * PREEMPT_RT.  So for RT it has to be done explicitly to protect
-	 * the sequence count write side critical section against a reader
-	 * or another writer preempting, which would result in a live lock.
-	 */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_disable();
+	preempt_disable_nested();
 	for (;;) {
 		unsigned n = dir->i_dir_seq;
 		if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
@@ -2618,8 +2610,7 @@ static inline void end_dir_add(struct inode *dir, unsigned int n,
 			       wait_queue_head_t *d_wait)
 {
 	smp_store_release(&dir->i_dir_seq, n + 2);
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		preempt_enable();
+	preempt_enable_nested();
 	wake_up_all(d_wait);
 }
 

[tip: sched/rt] preempt: Provide preempt_[dis|en]able_nested()

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the sched/rt branch of tip:

Commit-ID:     555bb4ccd1dd78d0263eae31629fe1fdd65c1fb5
Gitweb:        https://git.kernel.org/tip/555bb4ccd1dd78d0263eae31629fe1fdd65c1fb5
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Thu, 25 Aug 2022 18:41:24 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Mon, 19 Sep 2022 14:35:07 +02:00

preempt: Provide preempt_[dis|en]able_nested()

On PREEMPT_RT enabled kernels, spinlocks and rwlocks are neither disabling
preemption nor interrupts. Though there are a few places which depend on
the implicit preemption/interrupt disable of those locks, e.g. seqcount
write sections, per CPU statistics updates etc.

To avoid sprinkling CONFIG_PREEMPT_RT conditionals all over the place, add
preempt_disable_nested() and preempt_enable_nested() which should be
descriptive enough.

Add a lockdep assertion for the !PREEMPT_RT case to catch callers which
do not have preemption disabled.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220825164131.402717-2-bigeasy@linutronix.de

---
 include/linux/preempt.h | 42 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 42 insertions(+)

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index b4381f2..0df425b 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -421,4 +421,46 @@ static inline void migrate_enable(void) { }
 
 #endif /* CONFIG_SMP */
 
+/**
+ * preempt_disable_nested - Disable preemption inside a normally preempt disabled section
+ *
+ * Use for code which requires preemption protection inside a critical
+ * section which has preemption disabled implicitly on non-PREEMPT_RT
+ * enabled kernels, by e.g.:
+ *  - holding a spinlock/rwlock
+ *  - soft interrupt context
+ *  - regular interrupt handlers
+ *
+ * On PREEMPT_RT enabled kernels spinlock/rwlock held sections, soft
+ * interrupt context and regular interrupt handlers are preemptible and
+ * only prevent migration. preempt_disable_nested() ensures that preemption
+ * is disabled for cases which require CPU local serialization even on
+ * PREEMPT_RT. For non-PREEMPT_RT kernels this is a NOP.
+ *
+ * The use cases are code sequences which are not serialized by a
+ * particular lock instance, e.g.:
+ *  - seqcount write side critical sections where the seqcount is not
+ *    associated to a particular lock and therefore the automatic
+ *    protection mechanism does not work. This prevents a live lock
+ *    against a preempting high priority reader.
+ *  - RMW per CPU variable updates like vmstat.
+ */
+/* Macro to avoid header recursion hell vs. lockdep */
+#define preempt_disable_nested()				\
+do {								\
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))			\
+		preempt_disable();				\
+	else							\
+		lockdep_assert_preemption_disabled();		\
+} while (0)
+
+/**
+ * preempt_enable_nested - Undo the effect of preempt_disable_nested()
+ */
+static __always_inline void preempt_enable_nested(void)
+{
+	if (IS_ENABLED(CONFIG_PREEMPT_RT))
+		preempt_enable();
+}
+
 #endif /* __LINUX_PREEMPT_H */