linux-kernel - Re: [PATCH v3 1/5] mm: memcg: change flush_next_time to flush_last

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Message-ID: <CAF8kJuOh9rLBGBr3z2MXZgjjVbrSyFXQFCV+p1eC6DmKiir3Mw@mail.gmail.com>
Date:   Fri, 17 Nov 2023 10:27:44 -0800
From:   Chris Li <chrisl@...nel.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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>,
        Domenico Cerasuolo <cerasuolodomenico@...il.com>,
        linux-mm <linux-mm@...ck.org>, cgroups@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/5] mm: memcg: change flush_next_time to flush_last_time

Hi Yosry,

Acked-by: Chris Li <chrisl@...nel.org> (Google)

Chris

On Wed, Nov 15, 2023 at 6:24 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> flush_next_time is an inaccurate name. It's not the next time that
> periodic flushing will happen, it's rather the next time that
> ratelimited flushing can happen if the periodic flusher is late.
>
> Simplify its semantics by just storing the timestamp of the last flush
> instead, flush_last_time. Move the 2*FLUSH_TIME addition to
> mem_cgroup_flush_stats_ratelimited(), and add a comment explaining it.
> This way, all the ratelimiting semantics live in one place.
>
> No functional change intended.
>
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> Tested-by: Domenico Cerasuolo <cerasuolodomenico@...il.com>
> ---
>  mm/memcontrol.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 774bd6e21e278..18931d82f108f 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -593,7 +593,7 @@ 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_next_time;
> +static u64 flush_last_time;
>
>  #define FLUSH_TIME (2UL*HZ)
>
> @@ -653,7 +653,7 @@ static void do_flush_stats(void)
>             atomic_xchg(&stats_flush_ongoing, 1))
>                 return;
>
> -       WRITE_ONCE(flush_next_time, jiffies_64 + 2*FLUSH_TIME);
> +       WRITE_ONCE(flush_last_time, jiffies_64);
>
>         cgroup_rstat_flush(root_mem_cgroup->css.cgroup);
>
> @@ -669,7 +669,8 @@ void mem_cgroup_flush_stats(void)
>
>  void mem_cgroup_flush_stats_ratelimited(void)
>  {
> -       if (time_after64(jiffies_64, READ_ONCE(flush_next_time)))
> +       /* Only flush if the periodic flusher is one full cycle late */
> +       if (time_after64(jiffies_64, READ_ONCE(flush_last_time) + 2*FLUSH_TIME))
>                 mem_cgroup_flush_stats();
>  }
>
> --
> 2.43.0.rc0.421.g78406f8d94-goog
>
>