Re: [PATCH v2] mm: memcg: use rstat for non-hierarchical stats

[Yosry Ahmed]

On Tue, Jul 25, 2023 at 5:29 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> Currently, memcg uses rstat to maintain hierarchical stats. Counters are
> maintained for hierarchical stats at each memcg. Rstat tracks which
> cgroups have updates on which cpus to keep those counters fresh on the
> read-side.
>
> For non-hierarchical stats, we do not maintain counters. Instead, the
> percpu counters for a given stat need to be summed to get the
> non-hierarchical stat value. The original implementation did the same.
> At some point before rstat, non-hierarchical counters were introduced by
> commit a983b5ebee57 ("mm: memcontrol: fix excessive complexity in
> memory.stat reporting"). However, those counters were updated on the
> performance critical write-side, which caused regressions, so they were
> later removed by commit 815744d75152 ("mm: memcontrol: don't batch
> updates of local VM stats and events"). See [1] for more detailed
> history.
>
> Kernel versions in between a983b5ebee57 & 815744d75152 (a year and a
> half) enjoyed cheap reads of non-hierarchical stats, specifically on
> cgroup v1. When moving to more recent kernels, a performance regression
> for reading non-hierarchical stats is observed.
>
> Now that we have rstat, we know exactly which percpu counters have
> updates for each stat. We can maintain non-hierarchical counters again,
> making reads much more efficient, without affecting the performance
> critical write-side. Hence, add non-hierarchical (i.e local) counters
> for the stats, and extend rstat flushing to keep those up-to-date.
>
> A caveat is that we now a stats flush before reading
> local/non-hierarchical stats through {memcg/lruvec}_page_state_local()
> or memcg_events_local(), where we previously only needed a flush to
> read hierarchical stats. Most contexts reading non-hierarchical stats
> are already doing a flush, add a flush to the only missing context in
> count_shadow_nodes().
>
> With this patch, reading memory.stat from 1000 memcgs is 3x faster on a
> machine with 256 cpus on cgroup v1:
>  # for i in $(seq 1000); do mkdir /sys/fs/cgroup/memory/cg$i; done
>  # time cat /dev/cgroup/memory/cg*/memory.stat > /dev/null
>  real    0m0.125s
>  user    0m0.005s
>  sys     0m0.120s
>
> After:
>  real    0m0.032s
>  user    0m0.005s
>  sys     0m0.027s
>
> [1]https://lore.kernel.org/lkml/20230725201811.GA1231514@xxxxxxxxxxx/
>
> Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
>
> ---
>
> v1 -> v2:
> - Rewrite the changelog based on the history context provided by
>   Johannes (Thanks!).
> - Fix a subtle bug where updating a local counter would be missed if it
>   was cancelled out by a pending update from child memcgs.


Johannes, I fixed a subtle bug here and I kept your Ack, I wasn't sure
what the Ack retention policy should be here. A quick look at the fix
would be great.

Thanks!

>
>
> ---
>  include/linux/memcontrol.h |  7 ++--
>  mm/memcontrol.c            | 67 +++++++++++++++++++++-----------------
>  mm/workingset.c            |  1 +
>  3 files changed, 43 insertions(+), 32 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 5818af8eca5a..a9f2861a57a5 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -112,6 +112,9 @@ struct lruvec_stats {
>         /* Aggregated (CPU and subtree) state */
>         long state[NR_VM_NODE_STAT_ITEMS];
>
> +       /* Non-hierarchical (CPU aggregated) state */
> +       long state_local[NR_VM_NODE_STAT_ITEMS];
> +
>         /* Pending child counts during tree propagation */
>         long state_pending[NR_VM_NODE_STAT_ITEMS];
>  };
> @@ -1020,14 +1023,12 @@ static inline unsigned long lruvec_page_state_local(struct lruvec *lruvec,
>  {
>         struct mem_cgroup_per_node *pn;
>         long x = 0;
> -       int cpu;
>
>         if (mem_cgroup_disabled())
>                 return node_page_state(lruvec_pgdat(lruvec), idx);
>
>         pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> -       for_each_possible_cpu(cpu)
> -               x += per_cpu(pn->lruvec_stats_percpu->state[idx], cpu);
> +       x = READ_ONCE(pn->lruvec_stats.state_local[idx]);
>  #ifdef CONFIG_SMP
>         if (x < 0)
>                 x = 0;
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e8ca4bdcb03c..50f8035e998a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -742,6 +742,10 @@ struct memcg_vmstats {
>         long                    state[MEMCG_NR_STAT];
>         unsigned long           events[NR_MEMCG_EVENTS];
>
> +       /* Non-hierarchical (CPU aggregated) page state & events */
> +       long                    state_local[MEMCG_NR_STAT];
> +       unsigned long           events_local[NR_MEMCG_EVENTS];
> +
>         /* Pending child counts during tree propagation */
>         long                    state_pending[MEMCG_NR_STAT];
>         unsigned long           events_pending[NR_MEMCG_EVENTS];
> @@ -775,11 +779,8 @@ void __mod_memcg_state(struct mem_cgroup *memcg, int idx, int val)
>  /* idx can be of type enum memcg_stat_item or node_stat_item. */
>  static unsigned long memcg_page_state_local(struct mem_cgroup *memcg, int idx)
>  {
> -       long x = 0;
> -       int cpu;
> +       long x = READ_ONCE(memcg->vmstats->state_local[idx]);
>
> -       for_each_possible_cpu(cpu)
> -               x += per_cpu(memcg->vmstats_percpu->state[idx], cpu);
>  #ifdef CONFIG_SMP
>         if (x < 0)
>                 x = 0;
> @@ -926,16 +927,12 @@ static unsigned long memcg_events(struct mem_cgroup *memcg, int event)
>
>  static unsigned long memcg_events_local(struct mem_cgroup *memcg, int event)
>  {
> -       long x = 0;
> -       int cpu;
>         int index = memcg_events_index(event);
>
>         if (index < 0)
>                 return 0;
>
> -       for_each_possible_cpu(cpu)
> -               x += per_cpu(memcg->vmstats_percpu->events[index], cpu);
> -       return x;
> +       return READ_ONCE(memcg->vmstats->events_local[index]);
>  }
>
>  static void mem_cgroup_charge_statistics(struct mem_cgroup *memcg,
> @@ -5526,7 +5523,7 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu)
>         struct mem_cgroup *memcg = mem_cgroup_from_css(css);
>         struct mem_cgroup *parent = parent_mem_cgroup(memcg);
>         struct memcg_vmstats_percpu *statc;
> -       long delta, v;
> +       long delta, delta_cpu, v;
>         int i, nid;
>
>         statc = per_cpu_ptr(memcg->vmstats_percpu, cpu);
> @@ -5542,19 +5539,23 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu)
>                         memcg->vmstats->state_pending[i] = 0;
>
>                 /* Add CPU changes on this level since the last flush */
> +               delta_cpu = 0;
>                 v = READ_ONCE(statc->state[i]);
>                 if (v != statc->state_prev[i]) {
> -                       delta += v - statc->state_prev[i];
> +                       delta_cpu = v - statc->state_prev[i];
> +                       delta += delta_cpu;
>                         statc->state_prev[i] = v;
>                 }
>
> -               if (!delta)
> -                       continue;
> -
>                 /* Aggregate counts on this level and propagate upwards */
> -               memcg->vmstats->state[i] += delta;
> -               if (parent)
> -                       parent->vmstats->state_pending[i] += delta;
> +               if (delta_cpu)
> +                       memcg->vmstats->state_local[i] += delta_cpu;
> +
> +               if (delta) {
> +                       memcg->vmstats->state[i] += delta;
> +                       if (parent)
> +                               parent->vmstats->state_pending[i] += delta;
> +               }
>         }
>
>         for (i = 0; i < NR_MEMCG_EVENTS; i++) {
> @@ -5562,18 +5563,22 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu)
>                 if (delta)
>                         memcg->vmstats->events_pending[i] = 0;
>
> +               delta_cpu = 0;
>                 v = READ_ONCE(statc->events[i]);
>                 if (v != statc->events_prev[i]) {
> -                       delta += v - statc->events_prev[i];
> +                       delta_cpu = v - statc->events_prev[i];
> +                       delta += delta_cpu;
>                         statc->events_prev[i] = v;
>                 }
>
> -               if (!delta)
> -                       continue;
> +               if (delta_cpu)
> +                       memcg->vmstats->events_local[i] += delta_cpu;
>
> -               memcg->vmstats->events[i] += delta;
> -               if (parent)
> -                       parent->vmstats->events_pending[i] += delta;
> +               if (delta) {
> +                       memcg->vmstats->events[i] += delta;
> +                       if (parent)
> +                               parent->vmstats->events_pending[i] += delta;
> +               }
>         }
>
>         for_each_node_state(nid, N_MEMORY) {
> @@ -5591,18 +5596,22 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu)
>                         if (delta)
>                                 pn->lruvec_stats.state_pending[i] = 0;
>
> +                       delta_cpu = 0;
>                         v = READ_ONCE(lstatc->state[i]);
>                         if (v != lstatc->state_prev[i]) {
> -                               delta += v - lstatc->state_prev[i];
> +                               delta_cpu = v - lstatc->state_prev[i];
> +                               delta += delta_cpu;
>                                 lstatc->state_prev[i] = v;
>                         }
>
> -                       if (!delta)
> -                               continue;
> +                       if (delta_cpu)
> +                               pn->lruvec_stats.state_local[i] += delta_cpu;
>
> -                       pn->lruvec_stats.state[i] += delta;
> -                       if (ppn)
> -                               ppn->lruvec_stats.state_pending[i] += delta;
> +                       if (delta) {
> +                               pn->lruvec_stats.state[i] += delta;
> +                               if (ppn)
> +                                       ppn->lruvec_stats.state_pending[i] += delta;
> +                       }
>                 }
>         }
>  }
> diff --git a/mm/workingset.c b/mm/workingset.c
> index 4686ae363000..da58a26d0d4d 100644
> --- a/mm/workingset.c
> +++ b/mm/workingset.c
> @@ -664,6 +664,7 @@ static unsigned long count_shadow_nodes(struct shrinker *shrinker,
>                 struct lruvec *lruvec;
>                 int i;
>
> +               mem_cgroup_flush_stats();
>                 lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
>                 for (pages = 0, i = 0; i < NR_LRU_LISTS; i++)
>                         pages += lruvec_page_state_local(lruvec,
> --
> 2.41.0.487.g6d72f3e995-goog
>