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

From: Kees Cook <keescook@chromium•org>
To: Peter Zijlstra <peterz@infradead•org>
Cc: juri.lelli@redhat•com, mark.rutland@arm•com, heiko@sntech•de,
	geert+renesas@glider•be, catalin.marinas@arm•com,
	bsegall@google•com, will@kernel•org, mpe@ellerman•id.au,
	linux@armlinux•org.uk, dietmar.eggemann@arm•com,
	ben.dooks@codethink•co.uk, mgorman@suse•de, cl@rock-chips•com,
	huangtao@rock-chips•com, anshuman.khandual@arm•com,
	rostedt@goodmis•org, tglx@linutronix•de, surenb@google•com,
	mingo@redhat•com, allison@lohutok•net,
	linux-arm-kernel@lists•infradead.org, wad@chromium•org,
	gregkh@linuxfoundation•org, linux-kernel@vger•kernel.org,
	luto@amacapital•net, george_davis@mentor•com,
	sudeep.holla@arm•com, akpm@linux-foundation•org, info@metux•net,
	kstewart@linuxfoundation•org
Subject: Re: [PATCH v1 1/1] sched/fair: do not preempt current task if it is going to call schedule()
Date: Thu, 5 Mar 2020 09:30:30 -0800	[thread overview]
Message-ID: <202003050929.DD4DB3529@keescook> (raw)
In-Reply-To: <20200305095803.GW2596@hirez.programming.kicks-ass.net>

On Thu, Mar 05, 2020 at 10:58:03AM +0100, Peter Zijlstra wrote:
> On Thu, Mar 05, 2020 at 04:16:11PM +0800, cl@rock-chips•com wrote:
> > From: Liang Chen <cl@rock-chips•com>
> > 
> > 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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