linux-kernel - Re: [PATCH v2 4/9] cgroup: rstat: add WARN_ON

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Message-ID: <ZCU+8lSi+e4WgT3F@dhcp22.suse.cz>
Date:   Thu, 30 Mar 2023 09:49:06 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>
Cc:     Shakeel Butt <shakeelb@...gle.com>, Tejun Heo <tj@...nel.org>,
        Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>,
        Zefan Li <lizefan.x@...edance.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        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 4/9] cgroup: rstat: add WARN_ON_ONCE() if flushing
 outside task context

On Thu 30-03-23 00:13:16, Yosry Ahmed wrote:
> On Thu, Mar 30, 2023 at 12:06 AM Michal Hocko <mhocko@...e.com> wrote:
[...]
> > So the real question is. Do we really care so deeply? After all somebody
> > might be calling this from within a spin lock or irq disabled section
> > resulting in a similar situation without noticing.
> 
> There are discussions in [1] about making atomic rstat flush not
> disable irqs throughout the process, so in that case it would only
> result in a similar situation if the caller has irq disabled. The only
> caller that might have irq disabled is the same caller that might be
> in irq context before this series: mem_cgroup_usage().
> 
> On that note, and while I have your attention, I was wondering if we
> can eliminate the flush call completely from mem_cgroup_usage(), and
> read the global stats counters for root memcg instead of the root
> counters. There might be subtle differences, but the root memcg usage
> isn't super accurate now anyway (e.g. it doesn't include kernel
> memory).

root memcg stats are imprecise indeed and I have to admit I do not
really know why we are adding more work for that case. I have tried to
dig into git history for that yesterday but couldn't find a satisfying
answer. It goes all the way down to 2d146aa3aa842 which has done the
switch to rstat. Maybe Johannes remembers.

Anyway, back to this particular patch. I still do not see strong reasons
to be verbose about !in_task case so I would just drop this patch.
-- 
Michal Hocko
SUSE Labs