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

Libo Chen
2023-03-09T03:18:49Z

> On Jan 9, 2023, at 2:00 PM, Libo Chen <libo.chen@oracle.com> wrote:
>
> 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@oracle.com>
>>>> Signed-off-by: Libo Chen <libo.chen@oracle.com>
>>>> ---
>>>> 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.
>>

Hi Peter,

A second ping in case you missed the first one, what else should I do to get this fix in?

Libo

>>
>> Libo
>