Re: [PATCH] sched, fair: try to prevent migration thread from preempting non-cfs task

[Yafang Shao]

On Wed, Jun 16, 2021 at 4:35 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, Jun 15, 2021 at 08:15:51PM +0800, Yafang Shao wrote:
> > ---
> >  kernel/sched/fair.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 3248e24a90b0..597c7a940746 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -9797,6 +9797,20 @@ static int load_balance(int this_cpu, struct rq *this_rq,
> >                       /* Record that we found at least one task that could run on this_cpu */
> >                       env.flags &= ~LBF_ALL_PINNED;
> >
> > +                     /*
> > +                      * There may be a race between load balance starting migration
> > +                      * thread to pull the cfs running thread and the RT thread
> > +                      * waking up and preempting cfs task before migration threads
> > +                      * which then preempt the RT thread.
> > +                      * We'd better do the last minute check before starting
> > +                      * migration thread to avoid preempting latency-sensitive thread.
> > +                      */
> > +                     if (busiest->curr->sched_class != &fair_sched_class) {
> > +                             raw_spin_unlock_irqrestore(&busiest->lock,
> > +                                                        flags);
>
> This won't apply.
>
> Also, there's still a race window, you've just shrunk it, not fixed it.
> Busiest can reschedule between the mandatory rq unlock and doing the
> stopper wakeup.
>
> An actual fix might be to have the active migration done by a FIFO-1
> task, instead of stopper. The obvious down-side is that that would mean
> spawning yet another per-cpu kthread.
>

The stopper and the migration thread are different threads in the earlier days.
commit 969c79215a35 ("sched: replace migration_thread with cpu_stop")
merged them into one thread.

Regarding the priority of stopper (with highest priority) and
migration (higher than CFS, but lower than RT) , keeping them in one
single thread seems not a good way.


> How much do we care?



-- 
Thanks
Yafang