[PATCH v6 0/4] mm/memcg: Reduce kmemcache memory accounting overhead

[PATCH v6 0/4] mm/memcg: Reduce kmemcache memory accounting overhead

[Waiman Long]

 v6:
  - Rebased on top the latest Linus's tree.
  - Add additional comments to patches 2 and 3 as suggested by reviewers.
  - Rename the additional parameter in refill_obj_stock() to allow_uncharge
    to indicate if page uncharge is allowed or not.

 v5:
  - Combine v4 patches 2 & 4 into a single patch with minor twisting.
  - Move the user context stock access optimization patch to the end
    of the series.
  - Incorporate some changes suggested by Johannes.

 v4:
  - Drop v3 patch 1 as well as modification to mm/percpu.c as the percpu
    vmstat update isn't frequent enough to worth caching it.
  - Add a new patch 1 to move Move mod_objcg_state() to memcontrol.c instead.
  - Combine v3 patches 4 & 5 into a single patch (patch 3).
  - Add a new patch 4 to cache both reclaimable & unreclaimable vmstat updates.
  - Add a new patch 5 to improve refill_obj_stock() performance.

With the recent introduction of the new slab memory controller, we
eliminate the need for having separate kmemcaches for each memory
cgroup and reduce overall kernel memory usage. However, we also add
additional memory accounting overhead to each call of kmem_cache_alloc()
and kmem_cache_free().

For workloads that require a lot of kmemcache allocations and
de-allocations, they may experience performance regression as illustrated
in [1] and [2].

A simple kernel module that performs repeated loop of 100,000,000
kmem_cache_alloc() and kmem_cache_free() of either a small 32-byte
object or a big 4k object at module init time with a batch size of 4
(4 kmalloc's followed by 4 kfree's) is used for benchmarking. The
benchmarking tool was run on a kernel based on linux-next-20210419.
The test was run on a CascadeLake server with turbo-boosting disable
to reduce run-to-run variation.

The small object test exercises mainly the object stock charging and
vmstat update code paths. The large object test also exercises the
refill_obj_stock() and  __memcg_kmem_charge()/__memcg_kmem_uncharge()
code paths.

With memory accounting disabled, the run time was 3.130s with both small
object big object tests.

With memory accounting enabled, both cgroup v1 and v2 showed similar
results in the small object test.  The performance results of the large
object test, however, differed between cgroup v1 and v2.

The execution times with the application of various patches in the
patchset were:

  Applied patches   Run time   Accounting overhead   %age 1   %age 2
  ---------------   --------   -------------------   ------   ------

  Small 32-byte object:
       None          11.634s         8.504s          100.0%   271.7%
        1-2           9.425s         6.295s           74.0%   201.1%
        1-3           9.708s         6.578s           77.4%   210.2%
        1-4           8.062s         4.932s           58.0%   157.6%

  Large 4k object (v2):
       None          22.107s        18.977s          100.0%   606.3%
        1-2          20.960s        17.830s           94.0%   569.6%
        1-3          14.238s        11.108s           58.5%   354.9%
        1-4          11.329s         8.199s           43.2%   261.9%

  Large 4k object (v1):
       None          36.807s        33.677s          100.0%  1075.9%
        1-2          36.648s        33.518s           99.5%  1070.9%
        1-3          22.345s        19.215s           57.1%   613.9%
        1-4          18.662s        15.532s           46.1%   496.2%

  N.B. %age 1 = overhead/unpatched overhead
       %age 2 = overhead/accounting disabled time

Patch 2 (vmstat data stock caching) helps in both the small object test
and the large v2 object test. It doesn't help much in v1 big object test.

Patch 3 (refill_obj_stock improvement) does help the small object test
but offer significant performance improvement for the large object test
(both v1 and v2).

Patch 4 (eliminating irq disable/enable) helps in all test cases.

To test for the extreme case, a multi-threaded kmalloc/kfree
microbenchmark was run on the 2-socket 48-core 96-thread system with
96 testing threads in the same memcg doing kmalloc+kfree of a 4k object
with accounting enabled for 10s. The total number of kmalloc+kfree done
in kilo operations per second (kops/s) were as follows:

  Applied patches   v1 kops/s   v1 change   v2 kops/s   v2 change
  ---------------   ---------   ---------   ---------   ---------
       None           3,520        1.00X      6,242        1.00X
        1-2           4,304        1.22X      8,478        1.36X
        1-3           4,731        1.34X    418,142       66.99X
        1-4           4,587        1.30X    438,838       70.30X

With memory accounting disabled, the kmalloc/kfree rate was 1,481,291
kop/s. This test shows how significant the memory accouting overhead
can be in some extreme situations.

For this multithreaded test, the improvement from patch 2 mainly
comes from the conditional atomic xchg of objcg->nr_charged_bytes in
mod_objcg_state(). By using an unconditional xchg, the operation rates
were similar to the unpatched kernel.

Patch 3 elminates the single highly contended cacheline of
objcg->nr_charged_bytes for cgroup v2 leading to a huge performance
improvement. Cgroup v1, however, still has another highly contended
cacheline in the shared page counter &memcg->kmem. So the improvement
is only modest.

Patch 4 helps in cgroup v2, but performs worse in cgroup v1 as
eliminating the irq_disable/irq_enable overhead seems to aggravate the
cacheline contention.

[1] https://lore.kernel.org/linux-mm/20210408193948.vfktg3azh2wrt56t@gabell/T/#u
[2] https://lore.kernel.org/lkml/20210114025151.GA22932@xsang-OptiPlex-9020/

Waiman Long (4):
  mm/memcg: Move mod_objcg_state() to memcontrol.c
  mm/memcg: Cache vmstat data in percpu memcg_stock_pcp
  mm/memcg: Improve refill_obj_stock() performance
  mm/memcg: Optimize user context object stock access

 mm/memcontrol.c | 227 +++++++++++++++++++++++++++++++++++++++++-------
 mm/slab.h       |  16 +---
 2 files changed, 196 insertions(+), 47 deletions(-)

-- 
2.18.1

[PATCH v6 1/4] mm/memcg: Move mod_objcg_state() to memcontrol.c

[Waiman Long]

The mod_objcg_state() function is moved from mm/slab.h to mm/memcontrol.c
so that further optimization can be done to it in later patches without
exposing unnecessary details to other mm components.

Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Roman Gushchin <guro@fb.com>
---
 mm/memcontrol.c | 13 +++++++++++++
 mm/slab.h       | 16 ++--------------
 2 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c100265dc393..859f872b482e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -906,6 +906,19 @@ void __mod_lruvec_kmem_state(void *p, enum node_stat_item idx, int val)
 	rcu_read_unlock();
 }
 
+void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
+		     enum node_stat_item idx, int nr)
+{
+	struct mem_cgroup *memcg;
+	struct lruvec *lruvec;
+
+	rcu_read_lock();
+	memcg = obj_cgroup_memcg(objcg);
+	lruvec = mem_cgroup_lruvec(memcg, pgdat);
+	mod_memcg_lruvec_state(lruvec, idx, nr);
+	rcu_read_unlock();
+}
+
 /**
  * __count_memcg_events - account VM events in a cgroup
  * @memcg: the memory cgroup
diff --git a/mm/slab.h b/mm/slab.h
index 18c1927cd196..dcf964737d7e 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -239,6 +239,8 @@ static inline bool kmem_cache_debug_flags(struct kmem_cache *s, slab_flags_t fla
 #ifdef CONFIG_MEMCG_KMEM
 int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
 				 gfp_t gfp, bool new_page);
+void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
+		     enum node_stat_item idx, int nr);
 
 static inline void memcg_free_page_obj_cgroups(struct page *page)
 {
@@ -283,20 +285,6 @@ static inline bool memcg_slab_pre_alloc_hook(struct kmem_cache *s,
 	return true;
 }
 
-static inline void mod_objcg_state(struct obj_cgroup *objcg,
-				   struct pglist_data *pgdat,
-				   enum node_stat_item idx, int nr)
-{
-	struct mem_cgroup *memcg;
-	struct lruvec *lruvec;
-
-	rcu_read_lock();
-	memcg = obj_cgroup_memcg(objcg);
-	lruvec = mem_cgroup_lruvec(memcg, pgdat);
-	mod_memcg_lruvec_state(lruvec, idx, nr);
-	rcu_read_unlock();
-}
-
 static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s,
 					      struct obj_cgroup *objcg,
 					      gfp_t flags, size_t size,
-- 
2.18.1

[PATCH v6 2/4] mm/memcg: Cache vmstat data in percpu memcg_stock_pcp

[Waiman Long]

Before the new slab memory controller with per object byte charging,
charging and vmstat data update happen only when new slab pages are
allocated or freed. Now they are done with every kmem_cache_alloc()
and kmem_cache_free(). This causes additional overhead for workloads
that generate a lot of alloc and free calls.

The memcg_stock_pcp is used to cache byte charge for a specific
obj_cgroup to reduce that overhead. To further reducing it, this patch
makes the vmstat data cached in the memcg_stock_pcp structure as well
until it accumulates a page size worth of update or when other cached
data change. Caching the vmstat data in the per-cpu stock eliminates two
writes to non-hot cachelines for memcg specific as well as memcg-lruvecs
specific vmstat data by a write to a hot local stock cacheline.

On a 2-socket Cascade Lake server with instrumentation enabled and this
patch applied, it was found that about 20% (634400 out of 3243830)
of the time when mod_objcg_state() is called leads to an actual call
to __mod_objcg_state() after initial boot. When doing parallel kernel
build, the figure was about 17% (24329265 out of 142512465). So caching
the vmstat data reduces the number of calls to __mod_objcg_state()
by more than 80%.

Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
---
 mm/memcontrol.c | 90 +++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 87 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 859f872b482e..fbedfc55a248 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -906,8 +906,9 @@ void __mod_lruvec_kmem_state(void *p, enum node_stat_item idx, int val)
 	rcu_read_unlock();
 }
 
-void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
-		     enum node_stat_item idx, int nr)
+static inline void mod_objcg_mlstate(struct obj_cgroup *objcg,
+				     struct pglist_data *pgdat,
+				     enum node_stat_item idx, int nr)
 {
 	struct mem_cgroup *memcg;
 	struct lruvec *lruvec;
@@ -915,7 +916,7 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
 	rcu_read_lock();
 	memcg = obj_cgroup_memcg(objcg);
 	lruvec = mem_cgroup_lruvec(memcg, pgdat);
-	mod_memcg_lruvec_state(lruvec, idx, nr);
+	__mod_memcg_lruvec_state(lruvec, idx, nr);
 	rcu_read_unlock();
 }
 
@@ -2183,7 +2184,10 @@ struct memcg_stock_pcp {
 
 #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;
 #endif
 
 	struct work_struct work;
@@ -3132,6 +3136,67 @@ void __memcg_kmem_uncharge_page(struct page *page, int order)
 	obj_cgroup_put(objcg);
 }
 
+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;
+	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
+	 * changes.
+	 */
+	if (stock->cached_objcg != objcg) {
+		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;
+		stock->cached_objcg = objcg;
+		stock->cached_pgdat = pgdat;
+	} else if (stock->cached_pgdat != pgdat) {
+		/* Flush the existing cached vmstat data */
+		if (stock->nr_slab_reclaimable_b) {
+			mod_objcg_mlstate(objcg, pgdat, NR_SLAB_RECLAIMABLE_B,
+					  stock->nr_slab_reclaimable_b);
+			stock->nr_slab_reclaimable_b = 0;
+		}
+		if (stock->nr_slab_unreclaimable_b) {
+			mod_objcg_mlstate(objcg, pgdat, NR_SLAB_UNRECLAIMABLE_B,
+					  stock->nr_slab_unreclaimable_b);
+			stock->nr_slab_unreclaimable_b = 0;
+		}
+		stock->cached_pgdat = pgdat;
+	}
+
+	bytes = (idx == NR_SLAB_RECLAIMABLE_B) ? &stock->nr_slab_reclaimable_b
+					       : &stock->nr_slab_unreclaimable_b;
+	/*
+	 * Even for large object >= PAGE_SIZE, the vmstat data will still be
+	 * cached locally at least once before pushing it out.
+	 */
+	if (!*bytes) {
+		*bytes = nr;
+		nr = 0;
+	} else {
+		*bytes += nr;
+		if (abs(*bytes) > PAGE_SIZE) {
+			nr = *bytes;
+			*bytes = 0;
+		} else {
+			nr = 0;
+		}
+	}
+	if (nr)
+		mod_objcg_mlstate(objcg, pgdat, idx, nr);
+
+	local_irq_restore(flags);
+}
+
 static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
 {
 	struct memcg_stock_pcp *stock;
@@ -3179,6 +3244,25 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
 		stock->nr_bytes = 0;
 	}
 
+	/*
+	 * Flush the vmstat data in current stock
+	 */
+	if (stock->nr_slab_reclaimable_b || stock->nr_slab_unreclaimable_b) {
+		if (stock->nr_slab_reclaimable_b) {
+			mod_objcg_mlstate(old, stock->cached_pgdat,
+					  NR_SLAB_RECLAIMABLE_B,
+					  stock->nr_slab_reclaimable_b);
+			stock->nr_slab_reclaimable_b = 0;
+		}
+		if (stock->nr_slab_unreclaimable_b) {
+			mod_objcg_mlstate(old, stock->cached_pgdat,
+					  NR_SLAB_UNRECLAIMABLE_B,
+					  stock->nr_slab_unreclaimable_b);
+			stock->nr_slab_unreclaimable_b = 0;
+		}
+		stock->cached_pgdat = NULL;
+	}
+
 	obj_cgroup_put(old);
 	stock->cached_objcg = NULL;
 }
-- 
2.18.1

[PATCH v6 3/4] mm/memcg: Improve refill_obj_stock() performance

[Waiman Long]

There are two issues with the current refill_obj_stock() code. First of
all, when nr_bytes reaches over PAGE_SIZE, it calls drain_obj_stock() to
atomically flush out remaining bytes to obj_cgroup, clear cached_objcg
and do a obj_cgroup_put(). It is likely that the same obj_cgroup will
be used again which leads to another call to drain_obj_stock() and
obj_cgroup_get() as well as atomically retrieve the available byte from
obj_cgroup. That is costly. Instead, we should just uncharge the excess
pages, reduce the stock bytes and be done with it. The drain_obj_stock()
function should only be called when obj_cgroup changes.

Secondly, when charging an object of size not less than a page in
obj_cgroup_charge(), it is possible that the remaining bytes to be
refilled to the stock will overflow a page and cause refill_obj_stock()
to uncharge 1 page. To avoid the additional uncharge in this case, a new
allow_uncharge flag is added to refill_obj_stock() which will be set to
false when called from obj_cgroup_charge() so that an uncharge_pages()
call won't be issued right after a charge_pages() call unless the objcg
changes.

A multithreaded kmalloc+kfree microbenchmark on a 2-socket 48-core
96-thread x86-64 system with 96 testing threads were run.  Before this
patch, the total number of kilo kmalloc+kfree operations done for a 4k
large object by all the testing threads per second were 4,304 kops/s
(cgroup v1) and 8,478 kops/s (cgroup v2). After applying this patch, the
number were 4,731 (cgroup v1) and 418,142 (cgroup v2) respectively. This
represents a performance improvement of 1.10X (cgroup v1) and 49.3X
(cgroup v2).

Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
---
 mm/memcontrol.c | 48 +++++++++++++++++++++++++++++++++++-------------
 1 file changed, 35 insertions(+), 13 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index fbedfc55a248..513f3d56e89a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3281,10 +3281,12 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 	return false;
 }
 
-static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
+static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
+			     bool allow_uncharge)
 {
 	struct memcg_stock_pcp *stock;
 	unsigned long flags;
+	unsigned int nr_pages = 0;
 
 	local_irq_save(flags);
 
@@ -3293,14 +3295,21 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
 		drain_obj_stock(stock);
 		obj_cgroup_get(objcg);
 		stock->cached_objcg = objcg;
-		stock->nr_bytes = atomic_xchg(&objcg->nr_charged_bytes, 0);
+		stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes)
+				? atomic_xchg(&objcg->nr_charged_bytes, 0) : 0;
+		allow_uncharge = true;	/* Allow uncharge when objcg changes */
 	}
 	stock->nr_bytes += nr_bytes;
 
-	if (stock->nr_bytes > PAGE_SIZE)
-		drain_obj_stock(stock);
+	if (allow_uncharge && (stock->nr_bytes > PAGE_SIZE)) {
+		nr_pages = stock->nr_bytes >> PAGE_SHIFT;
+		stock->nr_bytes &= (PAGE_SIZE - 1);
+	}
 
 	local_irq_restore(flags);
+
+	if (nr_pages)
+		obj_cgroup_uncharge_pages(objcg, nr_pages);
 }
 
 int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
@@ -3312,14 +3321,27 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
 		return 0;
 
 	/*
-	 * In theory, memcg->nr_charged_bytes can have enough
+	 * In theory, objcg->nr_charged_bytes can have enough
 	 * pre-charged bytes to satisfy the allocation. However,
-	 * flushing memcg->nr_charged_bytes requires two atomic
-	 * operations, and memcg->nr_charged_bytes can't be big,
-	 * so it's better to ignore it and try grab some new pages.
-	 * memcg->nr_charged_bytes will be flushed in
-	 * refill_obj_stock(), called from this function or
-	 * independently later.
+	 * flushing objcg->nr_charged_bytes requires two atomic
+	 * operations, and objcg->nr_charged_bytes can't be big.
+	 * The shared objcg->nr_charged_bytes can also become a
+	 * performance bottleneck if all tasks of the same memcg are
+	 * trying to update it. So it's better to ignore it and try
+	 * grab some new pages. The stock's nr_bytes will be flushed to
+	 * objcg->nr_charged_bytes later on when objcg changes.
+	 *
+	 * The stock's nr_bytes may contain enough pre-charged bytes
+	 * to allow one less page from being charged, but we can't rely
+	 * on the pre-charged bytes not being changed outside of
+	 * consume_obj_stock() or refill_obj_stock(). So ignore those
+	 * pre-charged bytes as well when charging pages. To avoid a
+	 * page uncharge right after a page charge, we set the
+	 * allow_uncharge flag to false when calling refill_obj_stock()
+	 * to temporarily allow the pre-charged bytes to exceed the page
+	 * size limit. The maximum reachable value of the pre-charged
+	 * bytes is (sizeof(object) + PAGE_SIZE - 2) if there is no data
+	 * race.
 	 */
 	nr_pages = size >> PAGE_SHIFT;
 	nr_bytes = size & (PAGE_SIZE - 1);
@@ -3329,14 +3351,14 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size)
 
 	ret = obj_cgroup_charge_pages(objcg, gfp, nr_pages);
 	if (!ret && nr_bytes)
-		refill_obj_stock(objcg, PAGE_SIZE - nr_bytes);
+		refill_obj_stock(objcg, PAGE_SIZE - nr_bytes, false);
 
 	return ret;
 }
 
 void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
 {
-	refill_obj_stock(objcg, size);
+	refill_obj_stock(objcg, size, true);
 }
 
 #endif /* CONFIG_MEMCG_KMEM */
-- 
2.18.1

[PATCH v6 4/4] mm/memcg: Optimize user context object stock access

[Waiman Long]

Most kmem_cache_alloc() calls are from user context. With instrumentation
enabled, the measured amount of kmem_cache_alloc() calls from non-task
context was about 0.01% of the total.

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 sets of object stocks (in the new obj_stock structure)
for task context and interrupt context access respectively.

The task context object stock can be accessed after disabling preemption
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.

The downside of this change is that there are more data stored in local
object stocks and not reflected in the charge counter and the vmstat
arrays.  However, this is a small price to pay for better performance.

Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
---
 mm/memcontrol.c | 94 +++++++++++++++++++++++++++++++++++--------------
 1 file changed, 68 insertions(+), 26 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 513f3d56e89a..d8e90aa6e1ad 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -906,6 +906,10 @@ void __mod_lruvec_kmem_state(void *p, enum node_stat_item idx, int val)
 	rcu_read_unlock();
 }
 
+/*
+ * mod_objcg_mlstate() may be called with irq enabled, so
+ * mod_memcg_lruvec_state() should be used.
+ */
 static inline void mod_objcg_mlstate(struct obj_cgroup *objcg,
 				     struct pglist_data *pgdat,
 				     enum node_stat_item idx, int nr)
@@ -916,7 +920,7 @@ static inline void mod_objcg_mlstate(struct obj_cgroup *objcg,
 	rcu_read_lock();
 	memcg = obj_cgroup_memcg(objcg);
 	lruvec = mem_cgroup_lruvec(memcg, pgdat);
-	__mod_memcg_lruvec_state(lruvec, idx, nr);
+	mod_memcg_lruvec_state(lruvec, idx, nr);
 	rcu_read_unlock();
 }
 
@@ -2178,17 +2182,23 @@ void unlock_page_memcg(struct page *page)
 }
 EXPORT_SYMBOL(unlock_page_memcg);
 
-struct memcg_stock_pcp {
-	struct mem_cgroup *cached; /* this never be root cgroup */
-	unsigned int nr_pages;
-
+struct obj_stock {
 #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;
@@ -2198,12 +2208,12 @@ 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 memcg_stock_pcp *stock);
+static void drain_obj_stock(struct obj_stock *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 void drain_obj_stock(struct obj_stock *stock)
 {
 }
 static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
@@ -2213,6 +2223,40 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 }
 #endif
 
+/*
+ * 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())) {
+		preempt_disable();
+		stock = this_cpu_ptr(&memcg_stock);
+		return &stock->task_obj;
+	} else {
+		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);
+}
+
 /**
  * consume_stock: Try to consume stocked charge on this cpu.
  * @memcg: memcg to consume from.
@@ -2279,7 +2323,9 @@ static void drain_local_stock(struct work_struct *dummy)
 	local_irq_save(flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
-	drain_obj_stock(stock);
+	drain_obj_stock(&stock->irq_obj);
+	if (in_task())
+		drain_obj_stock(&stock->task_obj);
 	drain_stock(stock);
 	clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);
 
@@ -3139,13 +3185,10 @@ 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
@@ -3194,29 +3237,26 @@ 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);
+	put_obj_stock(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;
 	}
 
-	local_irq_restore(flags);
+	put_obj_stock(flags);
 
 	return ret;
 }
 
-static void drain_obj_stock(struct memcg_stock_pcp *stock)
+static void drain_obj_stock(struct obj_stock *stock)
 {
 	struct obj_cgroup *old = stock->cached_objcg;
 
@@ -3272,8 +3312,13 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 {
 	struct mem_cgroup *memcg;
 
-	if (stock->cached_objcg) {
-		memcg = obj_cgroup_memcg(stock->cached_objcg);
+	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 (memcg && mem_cgroup_is_descendant(memcg, root_memcg))
 			return true;
 	}
@@ -3284,13 +3329,10 @@ 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);
@@ -3306,7 +3348,7 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
 		stock->nr_bytes &= (PAGE_SIZE - 1);
 	}
 
-	local_irq_restore(flags);
+	put_obj_stock(flags);
 
 	if (nr_pages)
 		obj_cgroup_uncharge_pages(objcg, nr_pages);
-- 
2.18.1