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

[Chuyi Zhou]

Hello,

在 2024/7/22 15:16, Zhang Qiao 写道:
> hi
>
> 在 2024/7/22 14:04, Chuyi Zhou 写道:
> > 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.
>
> The cpu_cgroup_css_released() also doesn't seem to be in the cpu hotplug lock-holding context.

IIUC, cpus_read_lock/cpus_read_unlock can be called in 
cpu_cgroup_css_released() right? But cfs bandwidth destroy maybe run in 
a rcu callback since task group list is protected by RCU so we could not
get the lock. Did I miss something important?