linux-kernel - Re: [PATCH v3 3/5] mm: memcg: make stats flushing threshold per-memcg

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Message-ID: <CAJD7tkYnn6CxSJdo0QJ1hc6cFY_qWLuJ0=S6g_Pm=GBV+Ss-jw@mail.gmail.com>
Date:   Mon, 27 Nov 2023 13:13:44 -0800
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     kernel test robot <oliver.sang@...el.com>
Cc:     oe-lkp@...ts.linux.dev, lkp@...el.com,
        Johannes Weiner <hannes@...xchg.org>,
        Domenico Cerasuolo <cerasuolodomenico@...il.com>,
        cgroups@...r.kernel.org, linux-mm@...ck.org, ying.huang@...el.com,
        feng.tang@...el.com, fengwei.yin@...el.com,
        Andrew Morton <akpm@...ux-foundation.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-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/5] mm: memcg: make stats flushing threshold per-memcg

On Wed, Nov 22, 2023 at 5:54 AM kernel test robot <oliver.sang@...el.com> wrote:
>
>
>
> Hello,
>
> kernel test robot noticed a -30.2% regression of will-it-scale.per_thread_ops on:
>
>
> commit: c7fbfc7b4e089c4a9b292b1973a42a5761c1342f ("[PATCH v3 3/5] mm: memcg: make stats flushing threshold per-memcg")
> url: https://github.com/intel-lab-lkp/linux/commits/Yosry-Ahmed/mm-memcg-change-flush_next_time-to-flush_last_time/20231116-103300
> base: https://git.kernel.org/cgit/linux/kernel/git/akpm/mm.git mm-everything
> patch link: https://lore.kernel.org/all/20231116022411.2250072-4-yosryahmed@google.com/
> patch subject: [PATCH v3 3/5] mm: memcg: make stats flushing threshold per-memcg
>
> testcase: will-it-scale
> test machine: 104 threads 2 sockets (Skylake) with 192G memory
> parameters:
>
>         nr_task: 50%
>         mode: thread
>         test: fallocate2
>         cpufreq_governor: performance
>
>

This regression was also reported in v2, and I explicitly mention it
in the cover letter here:
https://lore.kernel.org/lkml/20231116022411.2250072-1-yosryahmed@google.com/

In a nutshell, I think this microbenchmark regression does not
represent real workloads. On the other hand, there are demonstrated
benefits on real workloads from this series in terms of stats reading
time.