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

[Libo Chen]

Hi Peter,

A gentle ping~ Vincent has signed it off. Let me know what else I should 
do for this patch.

Libo

On 8/15/22 12:19 PM, Libo Chen wrote:
> 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