Re: [PATCH v1 1/1] sched/fair: do not preempt current task if it is going to call schedule()

[Peter Zijlstra]

On Thu, Mar 05, 2020 at 04:16:11PM +0800, cl@xxxxxxxxxxxxxx wrote:
> From: Liang Chen <cl@xxxxxxxxxxxxxx>
> 
> when we create a kthread with ktrhead_create_on_cpu(),the child thread
> entry is ktread.c:ktrhead() which will be preempted by the parent after
> call complete(done) while schedule() is not called yet,then the parent
> will call wait_task_inactive(child) but the child is still on the runqueue,
> so the parent will schedule_hrtimeout() for 1 jiffy,it will waste a lot of
> time,especially on startup.
> 
>   parent                             child
> ktrhead_create_on_cpu()
>   wait_fo_completion(&done) -----> ktread.c:ktrhead()
>                              |----- complete(done);--wakeup and preempted by parent
>  kthread_bind() <------------|  |-> schedule();--dequeue here
>   wait_task_inactive(child)     |
>    schedule_hrtimeout(1 jiffy) -|
> 
> So we hope the child just wakeup parent but not preempted by parent, and the
> child is going to call schedule() soon,then the parent will not call
> schedule_hrtimeout(1 jiffy) as the child is already dequeue.
> 
> The same issue for ktrhead_park()&&kthread_parkme().
> This patch can save 120ms on rk312x startup with CONFIG_HZ=300.

> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index b262f47046ca..8a4e4c9cdc22 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -199,8 +199,10 @@ static void __kthread_parkme(struct kthread *self)
>  		if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags))
>  			break;
>  
> +		set_tsk_going_to_sched(current);
>  		complete(&self->parked);
>  		schedule();
> +		clear_tsk_going_to_sched(current);
>  	}
>  	__set_current_state(TASK_RUNNING);
>  }
> @@ -245,8 +247,10 @@ static int kthread(void *_create)
>  	/* OK, tell user we're spawned, wait for stop or wakeup */
>  	__set_current_state(TASK_UNINTERRUPTIBLE);
>  	create->result = current;
> +	set_tsk_going_to_sched(current);
>  	complete(done);
>  	schedule();
> +	clear_tsk_going_to_sched(current);
>  
>  	ret = -EINTR;
>  	if (!test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {

Were you looking for this? I think it does the same without having
fallen from the ugly tree...

diff --git a/kernel/kthread.c b/kernel/kthread.c
index b262f47046ca..62699ff414f4 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -199,8 +199,10 @@ static void __kthread_parkme(struct kthread *self)
 		if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags))
 			break;
 
+		preempt_disable()
 		complete(&self->parked);
-		schedule();
+		schedule_preempt_disabled();
+		preempt_enable();
 	}
 	__set_current_state(TASK_RUNNING);
 }
@@ -245,8 +247,10 @@ static int kthread(void *_create)
 	/* OK, tell user we're spawned, wait for stop or wakeup */
 	__set_current_state(TASK_UNINTERRUPTIBLE);
 	create->result = current;
+	preempt_disable()
 	complete(done);
-	schedule();
+	schedule_preempt_disabled();
+	preempt_enable();
 
 	ret = -EINTR;
 	if (!test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {