Re: [PATCH] mm: convert mm's rss stats into percpu_counter

[Yu Zhao <yuzhao@google.com>]

On Thu, Jun 8, 2023 at 5:14 AM Jan Kara <jack@suse.cz> wrote:
>
> On Mon 24-10-22 05:28:41, Shakeel Butt wrote:
> > Currently mm_struct maintains rss_stats which are updated on page fault
> > and the unmapping codepaths. For page fault codepath the updates are
> > cached per thread with the batch of TASK_RSS_EVENTS_THRESH which is 64.
> > The reason for caching is performance for multithreaded applications
> > otherwise the rss_stats updates may become hotspot for such
> > applications.
> >
> > However this optimization comes with the cost of error margin in the rss
> > stats. The rss_stats for applications with large number of threads can
> > be very skewed. At worst the error margin is (nr_threads * 64) and we
> > have a lot of applications with 100s of threads, so the error margin can
> > be very high. Internally we had to reduce TASK_RSS_EVENTS_THRESH to 32.
> >
> > Recently we started seeing the unbounded errors for rss_stats for
> > specific applications which use TCP rx0cp. It seems like
> > vm_insert_pages() codepath does not sync rss_stats at all.
> >
> > This patch converts the rss_stats into percpu_counter to convert the
> > error margin from (nr_threads * 64) to approximately (nr_cpus ^ 2).
> > However this conversion enable us to get the accurate stats for
> > situations where accuracy is more important than the cpu cost. Though
> > this patch does not make such tradeoffs.
> >
> > Signed-off-by: Shakeel Butt <shakeelb@google.com>
>
> Somewhat late to the game but our performance testing grid has noticed this
> commit causes a performance regression on shell-heavy workloads. For
> example running 'make test' in git sources on our test machine with 192
> CPUs takes about 4% longer, system time is increased by about 9%:
>
>                        before (9cd6ffa6025)  after (f1a7941243c1)
> Amean     User         471.12 *   0.30%*     481.77 *  -1.96%*
> Amean     System       244.47 *   0.90%*     269.13 *  -9.09%*
> Amean     Elapsed      709.22 *   0.45%*     742.27 *  -4.19%*
> Amean     CPU          100.00 (   0.20%)     101.00 *  -0.80%*
>
> Essentially this workload spawns in sequence a lot of short-lived tasks and
> the task startup + teardown cost is what this patch increases. To
> demonstrate this more clearly, I've written trivial (and somewhat stupid)
> benchmark shell_bench.sh:
>
> for (( i = 0; i < 20000; i++ )); do
>         /bin/true
> done
>
> And when run like:
>
> numactl -C 1 ./shell_bench.sh
>
> (I've forced physical CPU binding to avoid task migrating over the machine
> and cpu frequency scaling interfering which makes the numbers much more
> noisy) I get the following elapsed times:
>
>          9cd6ffa6025    f1a7941243c1
> Avg      6.807429       7.631571
> Stddev   0.021797       0.016483
>
> So some 12% regression in elapsed time. Just to be sure I've verified that
> per-cpu allocator patch [1] does not improve these numbers in any
> significant way.
>
> Where do we go from here? I think in principle the problem could be fixed
> by being clever and when the task has only a single thread, we don't bother
> with allocating pcpu counter (and summing it at the end) and just account
> directly in mm_struct. When the second thread is spawned, we bite the
> bullet, allocate pcpu counter and start with more scalable accounting.
> These shortlived tasks in shell workloads or similar don't spawn any
> threads so this should fix the regression. But this is obviously easier
> said than done...
>
>                                                                 Honza
>
> [1] https://lore.kernel.org/all/20230606125404.95256-1-yu.ma@intel.com/

Another regression reported earlier:
https://lore.kernel.org/linux-mm/202301301057.e55dad5b-oliver.sang@intel.com/