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

[Chengming Zhou]

On 2022/11/8 18:08, Peter Zijlstra wrote:
> On Tue, Nov 08, 2022 at 10:11:49AM +0100, Peter Zijlstra wrote:
>> On Mon, Nov 07, 2022 at 03:54:38PM +0000, Valentin Schneider wrote:
>>
>>> So that's the part for the p->sched_class->task_woken() callback, which
>>> only affects RT and DL (and only does something when !p->on_cpu). I *think*
>>> it's fine to remove it from ttwu_runnable() as any push/pull should have
>>> happened when other tasks were enqueued on the same CPU - with that said,
>>> it wouldn't hurt to double check this :-)
>>>
>>>
>>> As for the check_preempt_curr(), since per the above p can be preempted,
>>> you could have scenarios right now with CFS tasks where
>>> ttwu_runnable()->check_preempt_curr() causes NEED_RESCHED to be set.
>>>
>>> e.g. p0 does
>>>
>>>   set_current_state(TASK_UNINTERRUPTIBLE)
>>>
>>> but then gets interrupted by the tick, a p1 gets selected to run instead
>>> because of check_preempt_tick(), and then runs long enough to have
>>> check_preempt_curr() decide to let p0 preempt p1.
>>>
>>> That does require specific timing (lower tick frequency should make this
>>> more likely) and probably task niceness distribution too, but isn't
>>> impossible.
>>>
>>> Maybe try reading p->on_cpu, and only do the quick task state update if
>>> it's still the current task, otherwise do the preemption checks?
>>
>> I'm confused...
> 
> I am and Valentin has a point. It could indeed be preempted and in that
> case check_preempt_curr() could indeed make it get back on.
> 
> In that case his suggestion might make sense; something along the lines
> of so I suppose...
> 
> (And I think we can still do the reorg I proposed elsewhere, but I've not
> yet tried.)

Ok, thanks for these suggestions.

I will try to do this, do some tests and send v2.

Thanks.

> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index cb2aa2b54c7a..6944d9473295 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3698,9 +3698,16 @@ 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 (!p->on_cpu) {
> +			/*
> +			 * 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);