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

[Valentin Schneider]

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@xxxxxxxxxxxxx>
> ---
>  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.


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