[PATCH v2] mm: memcontrol: optimize per-lruvec stats counter memory usage

[PATCH v2] mm: memcontrol: optimize per-lruvec stats counter memory usage

[Muchun Song]

The vmstat threshold is 32 (MEMCG_CHARGE_BATCH), so the type of s32
of lruvec_stat_cpu is enough. And introduce struct per_cpu_lruvec_stat
to optimize memory usage.

The size of struct lruvec_stat is 304 bytes on 64 bits system. As it
is a per-cpu structure. So with this patch, we can save 304 / 2 * ncpu
bytes per-memcg per-node where ncpu is the number of the possible CPU.
If there are c memory cgroup (include dying cgroup) and n NUMA node in
the system. Finally, we can save (152 * ncpu * c * n) bytes.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
Changes in v1 -> v2:
 - Update the commit log to point out how many bytes that we can save.

 include/linux/memcontrol.h |  6 +++++-
 mm/memcontrol.c            | 10 +++++++++-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 3febf64d1b80..290d6ec8535a 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -92,6 +92,10 @@ struct lruvec_stat {
 	long count[NR_VM_NODE_STAT_ITEMS];
 };
 
+struct per_cpu_lruvec_stat {
+	s32 count[NR_VM_NODE_STAT_ITEMS];
+};
+
 /*
  * Bitmap of shrinker::id corresponding to memcg-aware shrinkers,
  * which have elements charged to this memcg.
@@ -111,7 +115,7 @@ struct mem_cgroup_per_node {
 	struct lruvec_stat __percpu *lruvec_stat_local;
 
 	/* Subtree VM stats (batched updates) */
-	struct lruvec_stat __percpu *lruvec_stat_cpu;
+	struct per_cpu_lruvec_stat __percpu *lruvec_stat_cpu;
 	atomic_long_t		lruvec_stat[NR_VM_NODE_STAT_ITEMS];
 
 	unsigned long		lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS];
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index eec44918d373..da6dc6ca388d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5198,7 +5198,7 @@ static int alloc_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
 		return 1;
 	}
 
-	pn->lruvec_stat_cpu = alloc_percpu_gfp(struct lruvec_stat,
+	pn->lruvec_stat_cpu = alloc_percpu_gfp(struct per_cpu_lruvec_stat,
 					       GFP_KERNEL_ACCOUNT);
 	if (!pn->lruvec_stat_cpu) {
 		free_percpu(pn->lruvec_stat_local);
@@ -7089,6 +7089,14 @@ static int __init mem_cgroup_init(void)
 {
 	int cpu, node;
 
+	/*
+	 * Currently s32 type (can refer to struct per_cpu_lruvec_stat) is
+	 * used for per-memcg-per-cpu caching of per-node statistics. In order
+	 * to work fine, we should make sure that the overfill threshold can't
+	 * exceed S32_MAX / PAGE_SIZE.
+	 */
+	BUILD_BUG_ON(MEMCG_CHARGE_BATCH > S32_MAX / PAGE_SIZE);
+
 	cpuhp_setup_state_nocalls(CPUHP_MM_MEMCQ_DEAD, "mm/memctrl:dead", NULL,
 				  memcg_hotplug_cpu_dead);
 
-- 
2.11.0

Re: [PATCH v2] mm: memcontrol: optimize per-lruvec stats counter memory usage

[Shakeel Butt]

On Tue, Dec 8, 2020 at 1:53 AM Muchun Song <songmuchun@bytedance.com> wrote:
>
> The vmstat threshold is 32 (MEMCG_CHARGE_BATCH), so the type of s32
> of lruvec_stat_cpu is enough. And introduce struct per_cpu_lruvec_stat
> to optimize memory usage.
>
> The size of struct lruvec_stat is 304 bytes on 64 bits system. As it
> is a per-cpu structure. So with this patch, we can save 304 / 2 * ncpu
> bytes per-memcg per-node where ncpu is the number of the possible CPU.
> If there are c memory cgroup (include dying cgroup) and n NUMA node in
> the system. Finally, we can save (152 * ncpu * c * n) bytes.
>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Few nits below:

Reviewed-by: Shakeel Butt <shakeelb@google.com>

> ---
> Changes in v1 -> v2:
>  - Update the commit log to point out how many bytes that we can save.
>
>  include/linux/memcontrol.h |  6 +++++-
>  mm/memcontrol.c            | 10 +++++++++-
>  2 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 3febf64d1b80..290d6ec8535a 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -92,6 +92,10 @@ struct lruvec_stat {
>         long count[NR_VM_NODE_STAT_ITEMS];
>  };
>
> +struct per_cpu_lruvec_stat {

lruvec_stat is also per-cpu, so the name per_cpu_lruvec_stat does not
really tell why it is different from lruvec. Maybe name is
batched_lruvec_stat or something else.

> +       s32 count[NR_VM_NODE_STAT_ITEMS];
> +};
> +
>  /*
>   * Bitmap of shrinker::id corresponding to memcg-aware shrinkers,
>   * which have elements charged to this memcg.
> @@ -111,7 +115,7 @@ struct mem_cgroup_per_node {
>         struct lruvec_stat __percpu *lruvec_stat_local;

A comment for the above why it still needs to be lruvec_stat.

>
>         /* Subtree VM stats (batched updates) */
> -       struct lruvec_stat __percpu *lruvec_stat_cpu;
> +       struct per_cpu_lruvec_stat __percpu *lruvec_stat_cpu;
>         atomic_long_t           lruvec_stat[NR_VM_NODE_STAT_ITEMS];
>
>         unsigned long           lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS];
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index eec44918d373..da6dc6ca388d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5198,7 +5198,7 @@ static int alloc_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
>                 return 1;
>         }
>
> -       pn->lruvec_stat_cpu = alloc_percpu_gfp(struct lruvec_stat,
> +       pn->lruvec_stat_cpu = alloc_percpu_gfp(struct per_cpu_lruvec_stat,
>                                                GFP_KERNEL_ACCOUNT);
>         if (!pn->lruvec_stat_cpu) {
>                 free_percpu(pn->lruvec_stat_local);
> @@ -7089,6 +7089,14 @@ static int __init mem_cgroup_init(void)
>  {
>         int cpu, node;
>
> +       /*
> +        * Currently s32 type (can refer to struct per_cpu_lruvec_stat) is
> +        * used for per-memcg-per-cpu caching of per-node statistics. In order
> +        * to work fine, we should make sure that the overfill threshold can't
> +        * exceed S32_MAX / PAGE_SIZE.
> +        */
> +       BUILD_BUG_ON(MEMCG_CHARGE_BATCH > S32_MAX / PAGE_SIZE);
> +
>         cpuhp_setup_state_nocalls(CPUHP_MM_MEMCQ_DEAD, "mm/memctrl:dead", NULL,
>                                   memcg_hotplug_cpu_dead);
>
> --
> 2.11.0
>

Re: [PATCH v2] mm: memcontrol: optimize per-lruvec stats counter memory usage

[Roman Gushchin]

On Tue, Dec 08, 2020 at 05:51:32PM +0800, Muchun Song wrote:
> The vmstat threshold is 32 (MEMCG_CHARGE_BATCH), so the type of s32
> of lruvec_stat_cpu is enough. And introduce struct per_cpu_lruvec_stat
> to optimize memory usage.
> 
> The size of struct lruvec_stat is 304 bytes on 64 bits system. As it
> is a per-cpu structure. So with this patch, we can save 304 / 2 * ncpu
> bytes per-memcg per-node where ncpu is the number of the possible CPU.
> If there are c memory cgroup (include dying cgroup) and n NUMA node in
> the system. Finally, we can save (152 * ncpu * c * n) bytes.

Honestly, I'm not convinced.
Say, ncpu = 32, n = 2, c = 500. We're saving <5Mb of memory.
If the machine has 128Gb of RAM, it's .000000003%.

Using longs (s64) allows not to think too much about overflows
and can also be slightly faster on 64-bit machines.

Thanks!

Re: [PATCH v2] mm: memcontrol: optimize per-lruvec stats counter memory usage

[Roman Gushchin]

On Tue, Dec 08, 2020 at 06:21:18PM -0800, Roman Gushchin wrote:
> On Tue, Dec 08, 2020 at 05:51:32PM +0800, Muchun Song wrote:
> > The vmstat threshold is 32 (MEMCG_CHARGE_BATCH), so the type of s32
> > of lruvec_stat_cpu is enough. And introduce struct per_cpu_lruvec_stat
> > to optimize memory usage.
> > 
> > The size of struct lruvec_stat is 304 bytes on 64 bits system. As it
> > is a per-cpu structure. So with this patch, we can save 304 / 2 * ncpu
> > bytes per-memcg per-node where ncpu is the number of the possible CPU.
> > If there are c memory cgroup (include dying cgroup) and n NUMA node in
> > the system. Finally, we can save (152 * ncpu * c * n) bytes.
> 
> Honestly, I'm not convinced.
> Say, ncpu = 32, n = 2, c = 500. We're saving <5Mb of memory.
> If the machine has 128Gb of RAM, it's .000000003%.

My bad, it's actually
(32*2*500*152)/(128*1024*1024*1024) = .0035%

but it's still in the noise.


> 
> Using longs (s64) allows not to think too much about overflows
> and can also be slightly faster on 64-bit machines.
> 
> Thanks!

Re: [External] Re: [PATCH v2] mm: memcontrol: optimize per-lruvec stats counter memory usage

[Muchun Song]

On Wed, Dec 9, 2020 at 10:21 AM Roman Gushchin <guro@fb.com> wrote:
>
> On Tue, Dec 08, 2020 at 05:51:32PM +0800, Muchun Song wrote:
> > The vmstat threshold is 32 (MEMCG_CHARGE_BATCH), so the type of s32
> > of lruvec_stat_cpu is enough. And introduce struct per_cpu_lruvec_stat
> > to optimize memory usage.
> >
> > The size of struct lruvec_stat is 304 bytes on 64 bits system. As it
> > is a per-cpu structure. So with this patch, we can save 304 / 2 * ncpu
> > bytes per-memcg per-node where ncpu is the number of the possible CPU.
> > If there are c memory cgroup (include dying cgroup) and n NUMA node in
> > the system. Finally, we can save (152 * ncpu * c * n) bytes.
>
> Honestly, I'm not convinced.
> Say, ncpu = 32, n = 2, c = 500. We're saving <5Mb of memory.
> If the machine has 128Gb of RAM, it's .000000003%.

Hi Roman,

When the cpu hotplug is enabled, the ncpu can be 256 on
some configurations. Also, the c can be more large when
there are many dying cgroup in the system.

So the savings depends on the environment and
configurations. Right?


>
> Using longs (s64) allows not to think too much about overflows
> and can also be slightly faster on 64-bit machines.
>
> Thanks!



--
Yours,
Muchun

Re: [External] Re: [PATCH v2] mm: memcontrol: optimize per-lruvec stats counter memory usage

[Roman Gushchin]

On Wed, Dec 09, 2020 at 10:31:55AM +0800, Muchun Song wrote:
> On Wed, Dec 9, 2020 at 10:21 AM Roman Gushchin <guro@fb.com> wrote:
> >
> > On Tue, Dec 08, 2020 at 05:51:32PM +0800, Muchun Song wrote:
> > > The vmstat threshold is 32 (MEMCG_CHARGE_BATCH), so the type of s32
> > > of lruvec_stat_cpu is enough.

Actually the threshold can be as big as MEMCG_CHARGE_BATCH * PAGE_SIZE.
It still fits into s32, but without explicitly saying it it's hard to
understand why not choosing s8, as in vmstat.c.

> > >
> > > The size of struct lruvec_stat is 304 bytes on 64 bits system. As it
> > > is a per-cpu structure. So with this patch, we can save 304 / 2 * ncpu
> > > bytes per-memcg per-node where ncpu is the number of the possible CPU.
> > > If there are c memory cgroup (include dying cgroup) and n NUMA node in
> > > the system. Finally, we can save (152 * ncpu * c * n) bytes.
> >
> > Honestly, I'm not convinced.
> > Say, ncpu = 32, n = 2, c = 500. We're saving <5Mb of memory.
> > If the machine has 128Gb of RAM, it's .000000003%.
> 
> Hi Roman,
> 
> When the cpu hotplug is enabled, the ncpu can be 256 on
> some configurations. Also, the c can be more large when
> there are many dying cgroup in the system.
> 
> So the savings depends on the environment and
> configurations. Right?

Of course, but machines with more CPUs tend to have more RAM as well.

Thanks!

Re: [External] Re: [PATCH v2] mm: memcontrol: optimize per-lruvec stats counter memory usage

[Muchun Song]

On Wed, Dec 9, 2020 at 11:52 AM Roman Gushchin <guro@fb.com> wrote:
>
> On Wed, Dec 09, 2020 at 10:31:55AM +0800, Muchun Song wrote:
> > On Wed, Dec 9, 2020 at 10:21 AM Roman Gushchin <guro@fb.com> wrote:
> > >
> > > On Tue, Dec 08, 2020 at 05:51:32PM +0800, Muchun Song wrote:
> > > > The vmstat threshold is 32 (MEMCG_CHARGE_BATCH), so the type of s32
> > > > of lruvec_stat_cpu is enough.
>
> Actually the threshold can be as big as MEMCG_CHARGE_BATCH * PAGE_SIZE.
> It still fits into s32, but without explicitly saying it it's hard to
> understand why not choosing s8, as in vmstat.c.

Yeah, here I need to update the commit log.

>
> > > >
> > > > The size of struct lruvec_stat is 304 bytes on 64 bits system. As it
> > > > is a per-cpu structure. So with this patch, we can save 304 / 2 * ncpu
> > > > bytes per-memcg per-node where ncpu is the number of the possible CPU.
> > > > If there are c memory cgroup (include dying cgroup) and n NUMA node in
> > > > the system. Finally, we can save (152 * ncpu * c * n) bytes.
> > >
> > > Honestly, I'm not convinced.
> > > Say, ncpu = 32, n = 2, c = 500. We're saving <5Mb of memory.
> > > If the machine has 128Gb of RAM, it's .000000003%.
> >
> > Hi Roman,
> >
> > When the cpu hotplug is enabled, the ncpu can be 256 on
> > some configurations. Also, the c can be more large when
> > there are many dying cgroup in the system.
> >
> > So the savings depends on the environment and
> > configurations. Right?
>
> Of course, but machines with more CPUs tend to have more RAM as well.

Here I mean possible CPU not online CPU. The number of possible
CPUs may be greater than online CPUs. The per-cpu allocator is based
on the number of possible CPUs. Right?

Thanks.

>
> Thanks!



-- 
Yours,
Muchun