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

From: Kees Cook <[email-masked]>
Date: Thu, 5 Mar 2020 09:30:30 -0800

On Thu, Mar 05, 2020 at 10:58:03AM +0100, Peter Zijlstra wrote:
> On Thu, Mar 05, 2020 at 04:16:11PM +0800, [email-masked] wrote:
> > From: Liang Chen <[email-masked]>
> > 
> > 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)) {

That's much nicer, yes! :) As I said, I don't know much about the
scheduler. ;)

-- 
Kees Cook

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