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Message-ID: <CAJD7tkYhg_mini5cEZq9SkkwOkThO77D=dbd=uW3NxJOE_b9kw@mail.gmail.com>
Date: Fri, 29 Sep 2023 12:57:11 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>,
Michal Hocko <mhocko@...nel.org>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Shakeel Butt <shakeelb@...gle.com>,
Muchun Song <muchun.song@...ux.dev>,
Ivan Babrou <ivan@...udflare.com>, Tejun Heo <tj@...nel.org>,
Michal Koutný <mkoutny@...e.com>,
Waiman Long <longman@...hat.com>, kernel-team@...udflare.com,
Wei Xu <weixugc@...gle.com>, Greg Thelen <gthelen@...gle.com>,
linux-mm@...ck.org, cgroups@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] mm: memcg: make stats flushing threshold per-memcg
On Thu, Sep 21, 2023 at 1:11 AM Yosry Ahmed <yosryahmed@...gle.com> 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.
Here is some additional testing. will-it-scale (specifically
per_process_ops in page_fault3 test) detected a 25.9% regression
before for a change in the stats update path, so I thought it would be
a good idea to run the numbers with this change. I ran all page_fault
tests in will-it-scale in a cgroup nested in a 4th level
($ROOT/a/b/c/d), as the work here scales with nesting, and 4 levels
under root seemed like a worst-ish case scenario. These are the
numbers from 30 runs (+ is good):
LABEL | MIN | MAX | MEAN
| MEDIAN | STDDEV |
------------------------------+-------+-------------+-------------+-------------+-------------+------------+
page_fault1_per_process_ops | | |
| | |
(A) base | 250644.000 | 298351.000 | 265207.738
| 262941.000 | 12112.379 |
(B) patched | 235885.000 | 276680.000 | 249249.191
| 248781.000 | 8767.457 |
| -5.89% | -7.26% | -6.02%
| -5.39% | |
page_fault1_per_thread_ops | | |
| | |
(A) base | 227214.000 | 271539.000 | 241618.484
| 240209.000 | 10162.207 |
(B) patched | 220156.000 | 248552.000 | 229820.671
| 229108.000 | 7506.582 |
| -3.11% | -8.47% | -4.88%
| -4.62% | |
page_fault1_scalability | | |
| | |
(A) base | 0.031175 | 0.038742 | 0.03545
| 0.035705 | 0.0015837 |
(B) patched | 0.026511 | 0.032529 | 0.029952
| 0.029957 | 0.0013551 |
| -9.65% | -9.21% | -9.29%
| -9.35% | |
page_fault2_per_process_ops | | |
| | |
(A) base | 197948.000 | 209020.000 | 203916.148
| 203496.000 | 2908.331 |
(B) patched | 183825.000 | 192870.000 | 186975.419
| 187023.000 | 1991.100 |
| -6.67% | -6.80% | -6.85%
| -6.90% | |
page_fault2_per_thread_ops | | |
| | |
(A) base | 166563.000 | 174843.000 | 170604.972
| 170532.000 | 1624.834 |
(B) patched | 161051.000 | 165887.000 | 163100.260
| 163263.000 | 1517.967 |
| -3.31% | -5.12% | -4.40%
| -4.26% | |
page_fault2_scalability | | |
| | |
(A) base | 0.052992 | 0.056427 | 0.054603
| 0.054693 | 0.00080196 |
(B) patched | 0.042582 | 0.046753 | 0.044882
| 0.044957 | 0.0011766 |
| +2.74% | -0.44% | -0.05%
| +0.33% | |
page_fault3_per_process_ops | | |
| | |
(A) base | 1282706.000 | 1323143.000 |
1299821.099 | 1297918.000 | 9882.872 |
(B) patched | 1232512.000 | 1269816.000 |
1248700.839 | 1247168.000 | 8454.891 |
| -3.91% | -4.03% | -3.93%
| -3.91% | |
page_fault3_per_thread_ops | | |
| | |
(A) base | 383583.000 | 390303.000 | 387216.963
| 387115.000 | 1605.760 |
(B) patched | 363791.000 | 373960.000 | 368538.213
| 368826.000 | 1852.594 |
| -5.16% | -4.19% | -4.82%
| -4.72% | |
page_fault3_scalability | | |
| | |
(A) base | 0.58206 | 0.62882 | 0.59909
| 0.59367 | 0.01256 |
(B) patched | 0.57967 | 0.62165 | 0.59995
| 0.59769 | 0.010088 |
| -0.41% | -1.14% | +0.14%
| +0.68% | |
Most numbers are within the range of normal variation of this
benchmark results or within the standard deviation (especially that
the fix for [1] assumes that 3% is noise -- and there were no further
practical complaints). These numbers are also from 4-level nesting,
which is more than most standard setups in my experience.
[1]https://lore.kernel.org/all/20190520063534.GB19312@shao2-debian/
>
> [1]https://github.com/google/neper
>
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> ---
> 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
>
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