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

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Date:   Thu, 9 Mar 2023 03:17:50 +0000
From:   Libo Chen <libo.chen@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     "mingo@...hat.com" <mingo@...hat.com>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "dietmar.eggemann@....com" <dietmar.eggemann@....com>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "linux-kernel@...r.kernel.org" <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 Jan 9, 2023, at 2:00 PM, Libo Chen <libo.chen@...cle.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@...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.
>> 

Hi Peter,

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


Libo 

>> 
>> Libo
>