Re: [PATCH v2 3/5] mm: memcg: make stats flushing threshold per-memcg

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, Oct 9, 2023 at 8:21 PM Yosry Ahmed <yosryahmed@google.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. The extra memory usage is insigificant.
>
> (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 results below show no 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. The following benchmarks were
> ran in a cgroup that is 4 levels deep (/sys/fs/cgroup/a/b/c/d), which is
> deeper than a usual setup:
>
> (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) "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.
>
> (3) will-it-scale page_fault tests. These tests (specifically
> per_process_ops in page_fault3 test) detected a 25.9% regression before
> for a change in the stats update path [2]. These are the
> numbers from 30 runs (+ is good):
>
>              LABEL            |     MEAN    |   MEDIAN    |   STDDEV   |
> ------------------------------+-------------+-------------+-------------
>   page_fault1_per_process_ops |             |             |            |
>   (A) base                    | 265207.738  | 262941.000  | 12112.379  |
>   (B) patched                 | 249249.191  | 248781.000  | 8767.457   |
>                               | -6.02%      | -5.39%      |            |
>   page_fault1_per_thread_ops  |             |             |            |
>   (A) base                    | 241618.484  | 240209.000  | 10162.207  |
>   (B) patched                 | 229820.671  | 229108.000  | 7506.582   |
>                               | -4.88%      | -4.62%      |            |
>   page_fault1_scalability     |             |             |
>   (A) base                    | 0.03545     | 0.035705    | 0.0015837  |
>   (B) patched                 | 0.029952    | 0.029957    | 0.0013551  |
>                               | -9.29%      | -9.35%      |            |
>   page_fault2_per_process_ops |             |             |
>   (A) base                    | 203916.148  | 203496.000  | 2908.331   |
>   (B) patched                 | 186975.419  | 187023.000  | 1991.100   |
>                               | -6.85%      | -6.90%      |            |
>   page_fault2_per_thread_ops  |             |             |
>   (A) base                    | 170604.972  | 170532.000  | 1624.834   |
>   (B) patched                 | 163100.260  | 163263.000  | 1517.967   |
>                               | -4.40%      | -4.26%      |            |
>   page_fault2_scalability     |             |             |
>   (A) base                    | 0.054603    | 0.054693    | 0.00080196 |
>   (B) patched                 | 0.044882    | 0.044957    | 0.0011766  |
>                               | -0.05%      | +0.33%      |            |
>   page_fault3_per_process_ops |             |             |
>   (A) base                    | 1299821.099 | 1297918.000 | 9882.872   |
>   (B) patched                 | 1248700.839 | 1247168.000 | 8454.891   |
>                               | -3.93%      | -3.91%      |            |
>   page_fault3_per_thread_ops  |             |             |
>   (A) base                    | 387216.963  | 387115.000  | 1605.760   |
>   (B) patched                 | 368538.213  | 368826.000  | 1852.594   |
>                               | -4.82%      | -4.72%      |            |
>   page_fault3_scalability     |             |             |
>   (A) base                    | 0.59909     | 0.59367     | 0.01256    |
>   (B) patched                 | 0.59995     | 0.59769     | 0.010088   |
>                               | +0.14%      | +0.68%      |            |
>
> There is some microbenchmarks regressions (and some minute improvements),
> but nothing outside the normal variance of this benchmark between kernel
> versions. The fix for [2]  assumes that 3% is noise -- and there were no
> further practical complaints), so hopefully this means that such variations
> in these microbenchmarks do not reflect on practical workloads.
>
> [1]https://github.com/google/neper
> [2]https://lore.kernel.org/all/20190520063534.GB19312@shao2-debian/
>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>

Johannes, as I mentioned in a reply to v1, I think this might be what
you suggested in our previous discussion [1], 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.

[1]https://lore.kernel.org/lkml/20230913153758.GB45543@cmpxchg.org/