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SubjectRe: [PATCH v5 1/2] sched/core: Minor optimize ttwu_runnable()
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Hello, ping :-)


On 2022/12/8 11:20, Chengming Zhou wrote:
> ttwu_runnable() is used as a fast wakeup path when the wakee task
> is running on CPU or runnable on RQ, in both cases we can just
> set its state to TASK_RUNNING to prevent a sleep.
>
> If the wakee task is on_cpu running, we don't need to update_rq_clock()
> or check_preempt_curr().
>
> But if the wakee task is on_rq && !on_cpu (e.g. an IRQ hit before
> the task got to schedule() and the task been preempted), we should
> check_preempt_curr() to see if it can preempt the current running.
>
> This also removes the class->task_woken() callback from ttwu_runnable(),
> which wasn't required per the RT/DL implementations: any required push
> operation would have been queued during class->set_next_task() when p
> got preempted.
>
> ttwu_runnable() also loses the update to rq->idle_stamp, as by definition
> the rq cannot be idle in this scenario.
>
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> Suggested-by: Valentin Schneider <vschneid@redhat.com>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Reviewed-by: Valentin Schneider <vschneid@redhat.com>
> ---
> v5:
> - Split the reorganization and optimization into two patches
> per Chen Yu. Thanks!
>
> v4:
> - s/This patch reorg/Reorganize/ per Bagas Sanjaya. Thanks!
>
> v3:
> - Improve the changelog per Valentin Schneider. Thanks!
>
> v2:
> - keep check_preempt_curr() for on_rq && !on_cpu case in ttwu_runnable(),
> per Valentin Schneider.
> - reorg ttwu_do_wakeup() and ttwu_do_activate() code, so ttwu_do_wakeup()
> can be reused in ttwu_runnable(), per Peter Zijlstra.
> - reuse ttwu_do_wakeup() in try_to_wake_up() (p == current) fast path too,
> so ttwu_do_wakeup() become the only place we mark task runnable.
> ---
> kernel/sched/core.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 314c2c0219d9..60aa7774e5f4 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3718,9 +3718,16 @@ 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);
> + if (!task_on_cpu(rq, p)) {
> + /*
> + * When on_rq && !on_cpu the task is preempted, see if
> + * it should preempt whatever is current there now.
> + */
> + update_rq_clock(rq);
> + check_preempt_curr(rq, p, wake_flags);
> + }
> + WRITE_ONCE(p->__state, TASK_RUNNING);
> + trace_sched_wakeup(p);
> ret = 1;
> }
> __task_rq_unlock(rq, &rf);

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