linux-kernel - Re: [PATCH 1/1] sched/fair: Fix inaccurate tally of ttwu_move

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Date:   Mon, 15 Aug 2022 12:19:01 -0700
From:   Libo Chen <libo.chen@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, vincent.guittot@...aro.org,
        juri.lelli@...hat.com, dietmar.eggemann@....com, mgorman@...e.de,
        linux-kernel@...r.kernel.org,
        Daniel Jordan <daniel.m.jordan@...cle.com>
Subject: Re: [PATCH 1/1] sched/fair: Fix inaccurate tally of ttwu_move_affine



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@...cle.com>
>> Signed-off-by: Libo Chen <libo.chen@...cle.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.


Libo