Re: [PATCH 1/1] sched/fair: Fix inaccurate tally of ttwu_move_affine

[Libo Chen]

On 8/15/22 04:01, Peter Zijlstra wrote:
> On Wed, Aug 10, 2022 at 03:33:13PM -0700, Libo Chen wrote:
> > There are scenarios where non-affine wakeups are incorrectly counted as
> > affine wakeups by schedstats.
> >
> > When wake_affine_idle() returns prev_cpu which doesn't equal to
> > nr_cpumask_bits, it will slip through the check: target == nr_cpumask_bits
> > in wake_affine() and be counted as if target == this_cpu in schedstats.
> >
> > Replace target == nr_cpumask_bits with target != this_cpu to make sure
> > affine wakeups are accurately tallied.
> >
> > Fixes: 806486c377e33 (sched/fair: Do not migrate if the prev_cpu is idle)
> > Suggested-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> > Signed-off-by: Libo Chen <libo.chen@xxxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index da388657d5ac..b179da4f8105 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6114,7 +6114,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
> >   		target = wake_affine_weight(sd, p, this_cpu, prev_cpu, sync);
> >   
> >   	schedstat_inc(p->stats.nr_wakeups_affine_attempts);
> > -	if (target == nr_cpumask_bits)
> > +	if (target != this_cpu)
> >   		return prev_cpu;
> >   
> >   	schedstat_inc(sd->ttwu_move_affine);
> This not only changes the accounting but also the placement, no?
No, it should only change the accounting. wake_affine() still returns 
prev_cpu if target equals to prev_cpu or nr_cpumask_bits, the same 
behavior as before.


Libo