Re: [PATCH] sched/core: Minor optimize ttwu_runnable()

[Chengming Zhou <zhouchengming@bytedance.com>]

On 2022/11/5 02:19, Valentin Schneider wrote:
> On 02/11/22 18:23, Chengming Zhou wrote:
>> ttwu_runnable() is used as a fast wakeup path when the wakee task
>> is between set_current_state() and schedule(), in which case all
>> we need to do is change p->state back to TASK_RUNNING. So we don't
>> need to update_rq_clock() and check_preempt_curr() in this case.
>>
>> Some performance numbers using mmtests/perfpipe on Intel Xeon server:
>>
>>                            linux-next                patched
>> Min       Time        8.67 (   0.00%)        8.66 (   0.13%)
>> 1st-qrtle Time        8.83 (   0.00%)        8.72 (   1.19%)
>> 2nd-qrtle Time        8.90 (   0.00%)        8.76 (   1.57%)
>> 3rd-qrtle Time        8.98 (   0.00%)        8.82 (   1.82%)
>> Max-1     Time        8.67 (   0.00%)        8.66 (   0.13%)
>> Max-5     Time        8.67 (   0.00%)        8.66 (   0.13%)
>> Max-10    Time        8.79 (   0.00%)        8.69 (   1.09%)
>> Max-90    Time        9.01 (   0.00%)        8.84 (   1.94%)
>> Max-95    Time        9.02 (   0.00%)        8.85 (   1.86%)
>> Max-99    Time        9.02 (   0.00%)        8.88 (   1.56%)
>> Max       Time        9.59 (   0.00%)        8.89 (   7.29%)
>> Amean     Time        8.92 (   0.00%)        8.77 *   1.65%*
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
>> ---
>>  kernel/sched/core.c | 5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 87c9cdf37a26..3785418de127 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -3718,9 +3718,8 @@ static int ttwu_runnable(struct task_struct *p, int wake_flags)
>>
>>       rq = __task_rq_lock(p, &rf);
>>       if (task_on_rq_queued(p)) {
>> -		/* check_preempt_curr() may use rq clock */
>> -		update_rq_clock(rq);
>> -		ttwu_do_wakeup(rq, p, wake_flags, &rf);
>> +		WRITE_ONCE(p->__state, TASK_RUNNING);
>> +		trace_sched_wakeup(p);
> 
> This also loses the class->task_woken() call, AFAICT we could have
> !p->on_cpu here (e.g. an IRQ hit before the task got to schedule()), but
> then again if there is a need to push we should have done that at the IRQ
> preempt via set_next_task_{rt, dl}()... So I'm starting to think this is
> OK, but that needs elaborating in the changelog.

Sorry, I don't get why we could have !p->on_cpu here?

I thought if we have task_on_rq_queued(p) here, it means p hasn't got to
__schedule() to deactivate_task(), so p should still be on_cpu?

Thanks.

> 
> 
>>               ret = 1;
>>       }
>>       __task_rq_unlock(rq, &rf);
>> --
>> 2.37.2
>