Re: [PATCH v2 1/2] cgroup: add BPF kfuncs to read a cpu cgroup's stats

Hi Tejun,

On Tue, Aug 18, 2026 at 12:47:11PM -1000, Tejun Heo wrote:
>On Tue, Aug 18, 2026 at 03:44:36PM -0700, Ziyang Men wrote:
>> > > +BTF_KFUNCS_START(bpf_rstat_common_kfunc_ids)
>> > > +BTF_ID_FLAGS(func, bpf_css_flush_rstat, KF_SLEEPABLE)
>> > > +BTF_ID_FLAGS(func, bpf_cgroup_base_stat, KF_SLEEPABLE)
>> >
>> > Why are these SLEEPABLE?
>> >
>>
>> The css_rstat_flush() calls might_sleep() and cond_resched().
>
>I see.
>
>> The bpf_cgroup_base_stat() takes an rstat spinlock_t, which can sleep on
>> PREEMPT_RT.
>
>Is this actually required? This doesn't really make sense to me. Shouldn't
>what SLEEPABLE mean change on RT kernels instead?

Oh sorry, I didn't notice that.

I might be wrong: this function calls the cputime_adjust(), which in turn
acquires raw_spin_lock_irqsave(), so there would be NMI deadlock in the
perf_event program. The __css_rstat_lock() take the spin_lock_irq() as well. So
maybe a SLEEPABLE tag is still necessary?

Please let me know your concerns. Thanks!

Best,
Ziyang

>
>Thanks.
>
>-- 
>tejun