linux-kernel - Re: [PATCH v2] cgroup: Add preemption protection to css_rstat

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Message-ID: <305559b6e8249a31ccbe1fe77fd3a3c041872c4b@linux.dev>
Date: Fri, 21 Nov 2025 06:21:01 +0000
From: "Jiayuan Chen" <jiayuan.chen@...ux.dev>
To: "Waiman Long" <llong@...hat.com>, cgroups@...r.kernel.org
Cc: tj@...nel.org, hannes@...xchg.org, mkoutny@...e.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] cgroup: Add preemption protection to
 css_rstat_updated()

November 21, 2025 at 13:07, "Waiman Long" <llong@...hat.com mailto:llong@...hat.com?to=%22Waiman%20Long%22%20%3Cllong%40redhat.com%3E > wrote:


> 
> On 11/20/25 11:06 PM, Jiayuan Chen wrote:
> 
> > 
> > BPF programs do not disable preemption, they only disable migration.
> >  Therefore, when running the cgroup_hierarchical_stats selftest, a
> >  warning [1] is generated.
> > 
> >  The css_rstat_updated() function is lockless and reentrant. However,
> >  as Tejun pointed out [2], preemption-related considerations need to
> >  be considered. Since css_rstat_updated() can be called from BPF where
> >  preemption is not disabled by its framework and it has already been
> >  exposed as a kfunc to BPF programs, introducing a new kfunc like bpf_xx
> >  will break existing uses. Thus, we directly make css_rstat_updated()
> >  preempt-safe here.
> > 
> My understand of Tejun's comment is to add bpf_preempt_disable() and bpf_preempt_enable() calls around the css_rstat_updated() call in the bpf program defined in tools/testing/selftests/bpf/prog_tests/cgroup_hierarchical_stats.c instead of adding that in the css_rstat_updated() function itself. But I may be wrong.
> 
> Cheers, Longman
>

If that's really the case, then I'd rather add a new wrapper kfunc for BPF
to replace css_rstat_updated(). Otherwise, whether it gets triggered would
depend entirely on users behavior.

Right now, this WARNING is showing up in all BPF selftests. Although it's not
treated as an error that fails the tests,it's visible in the action runs:
https://github.com/kernel-patches/bpf/actions