linux-kernel - Re: [PATCH v2 7/9] workingset: memcg: sleep when flushing stats in workingset

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Date:   Thu, 30 Mar 2023 00:42:36 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Tejun Heo <tj@...nel.org>, Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Koutný <mkoutny@...e.com>,
        Vasily Averin <vasily.averin@...ux.dev>,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH v2 7/9] workingset: memcg: sleep when flushing stats in workingset_refault()

On Thu, Mar 30, 2023 at 12:39 AM Michal Hocko <mhocko@...e.com> wrote:
>
> On Tue 28-03-23 22:16:42, Yosry Ahmed wrote:
> > In workingset_refault(), we call
> > mem_cgroup_flush_stats_atomic_ratelimited() to flush stats within an
> > RCU read section and with sleeping disallowed. Move the call above
> > the RCU read section and allow sleeping to avoid unnecessarily
> > performing a lot of work without sleeping.
>
> Could you say few words why the flushing is done before counters are
> updated rather than after (the RCU section)?

It's not about the counters that are updated, it's about the counters
that we read. Stats readers do a flush first to read accurate stats.
We flush before a read, not after an update.

> --
> Michal Hocko
> SUSE Labs