linux-kernel - Re: [PATCH v1 4/9] cgroup: rstat: add WARN_ON

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Date:   Tue, 28 Mar 2023 13:49:02 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Tejun Heo <tj@...nel.org>, Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>,
        Zefan Li <lizefan.x@...edance.com>,
        Michal Hocko <mhocko@...nel.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 v1 4/9] cgroup: rstat: add WARN_ON_ONCE() if flushing
 outside task context

On Tue, Mar 28, 2023 at 06:16:33AM +0000, Yosry Ahmed wrote:
> rstat flushing is too expensive to perform in irq context.
> The previous patch removed the only context that may invoke an rstat
> flush from irq context, add a WARN_ON_ONCE() to detect future
> violations, or those that we are not aware of.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> ---
>  kernel/cgroup/rstat.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> index d3252b0416b6..c2571939139f 100644
> --- a/kernel/cgroup/rstat.c
> +++ b/kernel/cgroup/rstat.c
> @@ -176,6 +176,8 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
>  {
>  	int cpu;
>  
> +	/* rstat flushing is too expensive for irq context */
> +	WARN_ON_ONCE(!in_task());
>  	lockdep_assert_held(&cgroup_rstat_lock);

This seems a bit arbitrary. Why is an irq caller forbidden, but an
irq-disabled, non-preemptible section caller is allowed? The latency
impact on the system would be the same, right?