linux-kernel - Re: [RFC PATCH 0/7] Make rstat flushing IRQ and sleep friendly

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Message-ID: <CAJD7tkYYQjZyDpTc02WTGG_aW+wS8m2k407X3NhLrN8Y_5RKPA@mail.gmail.com>
Date:   Wed, 22 Mar 2023 22:07:43 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Shakeel Butt <shakeelb@...gle.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>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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: [RFC PATCH 0/7] Make rstat flushing IRQ and sleep friendly

On Wed, Mar 22, 2023 at 9:10 PM Shakeel Butt <shakeelb@...gle.com> wrote:
>
> On Wed, Mar 22, 2023 at 9:00 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> >
> > Currently, if rstat flushing is invoked using the irqsafe variant
> > cgroup_rstat_flush_irqsafe(), we keep interrupts disabled and do not
> > sleep for the entire flush operation, which is O(# cpus * # cgroups).
> > This can be rather dangerous.
> >
> > Not all contexts that use cgroup_rstat_flush_irqsafe() actually cannot
> > sleep, and among those that cannot sleep, not all contexts require
> > interrupts to be disabled.
>
> Too many negations in the above sentence is making it very confusing.

Sorry, this is indeed very confusing. I guess a better rephrasing is:

Multiple code paths use cgroup_rstat_flush_irqsafe(), but many of them
can sleep. Even among the code paths that actually cannot sleep,
multiple ones are interruptible.