Re: [PATCH 3/5] mm: memcg: make stats flushing threshold per-memcg

[Yosry Ahmed]

On Thu, Sep 21, 2023 at 1:11 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> A global counter for the magnitude of memcg stats update is maintained
> on the memcg side to avoid invoking rstat flushes when the pending
> updates are not significant. This avoids unnecessary flushes, which are
> not very cheap even if there isn't a lot of stats to flush. It also
> avoids unnecessary lock contention on the underlying global rstat lock.
>
> Make this threshold per-memcg. The scheme is followed where percpu (now
> also per-memcg) counters are incremented in the update path, and only
> propagated to per-memcg atomics when they exceed a certain threshold.
>
> This provides two benefits:
> (a) On large machines with a lot of memcgs, the global threshold can be
> reached relatively fast, so guarding the underlying lock becomes less
> effective. Making the threshold per-memcg avoids this.
>
> (b) Having a global threshold makes it hard to do subtree flushes, as we
> cannot reset the global counter except for a full flush. Per-memcg
> counters removes this as a blocker from doing subtree flushes, which
> helps avoid unnecessary work when the stats of a small subtree are
> needed.
>
> Nothing is free, of course. This comes at a cost:
> (a) A new per-cpu counter per memcg, consuming NR_CPUS * NR_MEMCGS * 4
> bytes.
>
> (b) More work on the update side, although in the common case it will
> only be percpu counter updates. The amount of work scales with the
> number of ancestors (i.e. tree depth). This is not a new concept, adding
> a cgroup to the rstat tree involves a parent loop, so is charging.
> Testing in a later patch shows this doesn't introduce significant
> regressions.
>
> (c) The error margin in the stats for the system as a whole increases
> from NR_CPUS * MEMCG_CHARGE_BATCH to NR_CPUS * MEMCG_CHARGE_BATCH *
> NR_MEMCGS. This is probably fine because we have a similar per-memcg
> error in charges coming from percpu stocks, and we have a periodic
> flusher that makes sure we always flush all the stats every 2s anyway.
>
> This patch was tested to make sure no significant regressions are
> introduced on the update path as follows. In a cgroup that is 4 levels
> deep (/sys/fs/cgroup/a/b/c/d), the following benchmarks were ran:
>
> (a) neper [1] with 1000 flows and 100 threads (single machine). The
> values in the table are the average of server and client throughputs in
> mbps after 30 iterations, each running for 30s:
>
>                                 tcp_rr          tcp_stream
> Base                            9504218.56      357366.84
> Patched                         9656205.68      356978.39
> Delta                           +1.6%           -0.1%
> Standard Deviation              0.95%           1.03%
>
> An increase in the performance of tcp_rr doesn't really make sense, but
> it's probably in the noise. The same tests were ran with 1 flow and 1
> thread but the throughput was too noisy to make any conclusions (the
> averages did not show regressions nonetheless).
>
> Looking at perf for one iteration of the above test, __mod_memcg_state()
> (which is where memcg_rstat_updated() is called) does not show up at all
> without this patch, but it shows up with this patch as 1.06% for tcp_rr
> and 0.36% for tcp_stream.
>
> (b) Running "stress-ng --vm 0 -t 1m --times --perf". I don't understand
> stress-ng very well, so I am not sure that's the best way to test this,
> but it spawns 384 workers and spits a lot of metrics which looks nice :)
> I picked a few ones that seem to be relevant to the stats update path. I
> also included cache misses as this patch introduce more atomics that may
> bounce between cpu caches:
>
> Metric                  Base            Patched         Delta
> Cache Misses            3.394 B/sec     3.433 B/sec     +1.14%
> Cache L1D Read          0.148 T/sec     0.154 T/sec     +4.05%
> Cache L1D Read Miss     20.430 B/sec    21.820 B/sec    +6.8%
> Page Faults Total       4.304 M/sec     4.535 M/sec     +5.4%
> Page Faults Minor       4.304 M/sec     4.535 M/sec     +5.4%
> Page Faults Major       18.794 /sec     0.000 /sec
> Kmalloc                 0.153 M/sec     0.152 M/sec     -0.65%
> Kfree                   0.152 M/sec     0.153 M/sec     +0.65%
> MM Page Alloc           4.640 M/sec     4.898 M/sec     +5.56%
> MM Page Free            4.639 M/sec     4.897 M/sec     +5.56%
> Lock Contention Begin   0.362 M/sec     0.479 M/sec     +32.32%
> Lock Contention End     0.362 M/sec     0.479 M/sec     +32.32%
> page-cache add          238.057 /sec    0.000 /sec
> page-cache del          6.265 /sec      6.267 /sec      -0.03%
>
> This is only using a single run in each case. I am not sure what to
> make out of most of these numbers, but they mostly seem in the noise
> (some better, some worse). The lock contention numbers are interesting.
> I am not sure if higher is better or worse here. No new locks or lock
> sections are introduced by this patch either way.
>
> Looking at perf, __mod_memcg_state() shows up as 0.00% with and without
> this patch. This is suspicious, but I verified while stress-ng is
> running that all the threads are in the right cgroup.
>
> [1]https://github.com/google/neper
>
> Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>


Johannes, I think this might be what you have suggested in our
previous discussion, but I am not sure this is what you meant for the
update path, so I did not add a Suggested-by. Please let me know if
this is what you meant and I can amend the tag as such.

>
> ---
>  mm/memcontrol.c | 49 +++++++++++++++++++++++++++++++++----------------
>  1 file changed, 33 insertions(+), 16 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index ef7ad66a9e4c..c273c65bb642 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -627,6 +627,9 @@ struct memcg_vmstats_percpu {
>         /* Cgroup1: threshold notifications & softlimit tree updates */
>         unsigned long           nr_page_events;
>         unsigned long           targets[MEM_CGROUP_NTARGETS];
> +
> +       /* Stats updates since the last flush */
> +       unsigned int            stats_updates;
>  };
>
>  struct memcg_vmstats {
> @@ -641,6 +644,9 @@ struct memcg_vmstats {
>         /* Pending child counts during tree propagation */
>         long                    state_pending[MEMCG_NR_STAT];
>         unsigned long           events_pending[NR_MEMCG_EVENTS];
> +
> +       /* Stats updates since the last flush */
> +       atomic64_t              stats_updates;
>  };
>
>  /*
> @@ -660,9 +666,7 @@ struct memcg_vmstats {
>   */
>  static void flush_memcg_stats_dwork(struct work_struct *w);
>  static DECLARE_DEFERRABLE_WORK(stats_flush_dwork, flush_memcg_stats_dwork);
> -static DEFINE_PER_CPU(unsigned int, stats_updates);
>  static atomic_t stats_flush_ongoing = ATOMIC_INIT(0);
> -static atomic_t stats_flush_threshold = ATOMIC_INIT(0);
>  static u64 flush_last_time;
>
>  #define FLUSH_TIME (2UL*HZ)
> @@ -689,26 +693,37 @@ static void memcg_stats_unlock(void)
>         preempt_enable_nested();
>  }
>
> +
> +static bool memcg_should_flush_stats(struct mem_cgroup *memcg)
> +{
> +       return atomic64_read(&memcg->vmstats->stats_updates) >
> +               MEMCG_CHARGE_BATCH * num_online_cpus();
> +}
> +
>  static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
>  {
> +       int cpu = smp_processor_id();
>         unsigned int x;
>
>         if (!val)
>                 return;
>
> -       cgroup_rstat_updated(memcg->css.cgroup, smp_processor_id());
> +       cgroup_rstat_updated(memcg->css.cgroup, cpu);
> +
> +       for (; memcg; memcg = parent_mem_cgroup(memcg)) {
> +               x = __this_cpu_add_return(memcg->vmstats_percpu->stats_updates,
> +                                         abs(val));
> +
> +               if (x < MEMCG_CHARGE_BATCH)
> +                       continue;
>
> -       x = __this_cpu_add_return(stats_updates, abs(val));
> -       if (x > MEMCG_CHARGE_BATCH) {
>                 /*
> -                * If stats_flush_threshold exceeds the threshold
> -                * (>num_online_cpus()), cgroup stats update will be triggered
> -                * in __mem_cgroup_flush_stats(). Increasing this var further
> -                * is redundant and simply adds overhead in atomic update.
> +                * If @memcg is already flush-able, increasing stats_updates is
> +                * redundant. Avoid the overhead of the atomic update.
>                  */
> -               if (atomic_read(&stats_flush_threshold) <= num_online_cpus())
> -                       atomic_add(x / MEMCG_CHARGE_BATCH, &stats_flush_threshold);
> -               __this_cpu_write(stats_updates, 0);
> +               if (!memcg_should_flush_stats(memcg))
> +                       atomic64_add(x, &memcg->vmstats->stats_updates);
> +               __this_cpu_write(memcg->vmstats_percpu->stats_updates, 0);
>         }
>  }
>
> @@ -727,13 +742,12 @@ static void do_flush_stats(void)
>
>         cgroup_rstat_flush(root_mem_cgroup->css.cgroup);
>
> -       atomic_set(&stats_flush_threshold, 0);
>         atomic_set(&stats_flush_ongoing, 0);
>  }
>
>  void mem_cgroup_flush_stats(void)
>  {
> -       if (atomic_read(&stats_flush_threshold) > num_online_cpus())
> +       if (memcg_should_flush_stats(root_mem_cgroup))
>                 do_flush_stats();
>  }
>
> @@ -747,8 +761,8 @@ void mem_cgroup_flush_stats_ratelimited(void)
>  static void flush_memcg_stats_dwork(struct work_struct *w)
>  {
>         /*
> -        * Always flush here so that flushing in latency-sensitive paths is
> -        * as cheap as possible.
> +        * Deliberately ignore memcg_should_flush_stats() here so that flushing
> +        * in latency-sensitive paths is as cheap as possible.
>          */
>         do_flush_stats();
>         queue_delayed_work(system_unbound_wq, &stats_flush_dwork, FLUSH_TIME);
> @@ -5622,6 +5636,9 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu)
>                         }
>                 }
>         }
> +       /* We are in a per-cpu loop here, only do the atomic write once */
> +       if (atomic64_read(&memcg->vmstats->stats_updates))
> +               atomic64_set(&memcg->vmstats->stats_updates, 0);
>  }
>
>  #ifdef CONFIG_MMU
> --
> 2.42.0.459.ge4e396fd5e-goog
>