Re: [PATCH 1/2] sched/fair: Decrease cfs bandwidth usage in task_group destruction

[Chuyi Zhou]

Hello

在 2024/7/22 11:47, Zhang Qiao 写道:
> Hi, Chuyi
>
> 在 2024/7/21 20:52, Chuyi Zhou 写道:
> > The static key __cfs_bandwidth_used is used to indicate whether bandwidth
> > control is enabled in the system. Currently, it is only decreased when a
> > task group disables bandwidth control. This is incorrect because if there
> > was a task group in the past that enabled bandwidth control, the
> > __cfs_bandwidth_used will never go to zero, even if there are no task_group
> > using bandwidth control now.
> >
> > This patch tries to fix this issue by decrsasing bandwidth usage in
> > destroy_cfs_bandwidth().
> >
> > Signed-off-by: Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index b1e07ce90284..7ad50dc31a93 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6447,6 +6447,9 @@ static void destroy_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
> >   	hrtimer_cancel(&cfs_b->period_timer);
> >   	hrtimer_cancel(&cfs_b->slack_timer);
> >   
> > +	if (cfs_b->quota != RUNTIME_INF)
> > +		cfs_bandwidth_usage_dec();
>
> This calls static_key_slow_dec_cpuslocked, but destroy_cfs_bandwidth
> isn't holding the hotplug lock [1].
>
> For fixing this issue, i also sent a patch, but it be not merged into mainline [2].
>
> [1]: https://lore.kernel.org/all/20210712162655.w3j6uczwbfkzazvt@xxxxxxxxxx/
> [2]: https://lore.kernel.org/all/20210910094139.184582-1-zhangqiao22@xxxxxxxxxx/

Thanks for your information.

I think maybe cfs_bandwidth_usage_dec() should be moved to other more 
suitable places where could hold hotplug lock(e.g. 
cpu_cgroup_css_released()). I would do some test to verify it.

Thanks.