linux-kernel - Re: [patchlet] Re: Scheduler bug related to rq->skip_clock

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Date:	Tue, 07 Dec 2010 17:41:21 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <efault@....de>
Cc:	Yong Zhang <yong.zhang0@...il.com>,
	"Bjoern B. Brandenburg" <bbb.lst@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrea Bastoni <bastoni@...g.uniroma2.it>,
	"James H. Anderson" <anderson@...unc.edu>,
	linux-kernel@...r.kernel.org
Subject: Re: [patchlet] Re: Scheduler bug related to rq->skip_clock_update?

On Mon, 2010-12-06 at 09:32 +0100, Mike Galbraith wrote:

>  kernel/fork.c  |    1 +
>  kernel/sched.c |    6 +++---
>  2 files changed, 4 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6.37.git/kernel/sched.c
> ===================================================================
> --- linux-2.6.37.git.orig/kernel/sched.c
> +++ linux-2.6.37.git/kernel/sched.c
> @@ -660,6 +660,7 @@ inline void update_rq_clock(struct rq *r
>  
>  		sched_irq_time_avg_update(rq, irq_time);
>  	}
> +	rq->skip_clock_update = 0;
>  }
>  
>  /*

Shouldn't we do that at the end of schedule()? Since the purpose of
->skip_clock_update is to avoid multiple calls to:
  - avoid overhead
  - ensure scheduling is accounted at a single point

[ for that latter purpose it might also make sense to put that point
somewhere around context_switch() but due to the fact that we need a
clock update early that's a bit impractical. ]

Hmm?

> @@ -2138,7 +2139,7 @@ static void check_preempt_curr(struct rq
>  	 * A queue event has occurred, and we're going to schedule.  In
>  	 * this case, we can save a useless back to back clock update.
>  	 */
> -	if (test_tsk_need_resched(rq->curr))
> +	if (rq->curr->se.on_rq && test_tsk_need_resched(rq->curr))
>  		rq->skip_clock_update = 1;
>  }

OK, I initially tried to replace the test with a return value of
->check_preempt_curr() and such, but that turns into a lot of code and
won't necessarily be any better.

> @@ -3854,7 +3855,6 @@ static void put_prev_task(struct rq *rq,
>  {
>  	if (prev->se.on_rq)
>  		update_rq_clock(rq);
> -	rq->skip_clock_update = 0;
>  	prev->sched_class->put_prev_task(rq, prev);
>  }

See the first note.

> @@ -3912,7 +3912,6 @@ need_resched_nonpreemptible:
>  		hrtick_clear(rq);
>  
>  	raw_spin_lock_irq(&rq->lock);
> -	clear_tsk_need_resched(prev);
>  
>  	switch_count = &prev->nivcsw;
>  	if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
> @@ -3942,6 +3941,7 @@ need_resched_nonpreemptible:
>  	if (unlikely(!rq->nr_running))
>  		idle_balance(cpu, rq);
>  
> +	clear_tsk_need_resched(prev);
>  	put_prev_task(rq, prev);
>  	next = pick_next_task(rq);

Good find, this needs to be done after the idle balancing because that
can release the rq->lock and allow for TIF_NEED_RESCHED to be set again.

Maybe complement this with a WARN_ON_ONCE(test_tsk_need_resched(next))
somewhere after pick_next_task() so as to ensure that !current has !
TIF_NEED_RESCHED.

> Index: linux-2.6.37.git/kernel/fork.c
> ===================================================================
> --- linux-2.6.37.git.orig/kernel/fork.c
> +++ linux-2.6.37.git/kernel/fork.c
> @@ -275,6 +275,7 @@ static struct task_struct *dup_task_stru
>  
>  	setup_thread_stack(tsk, orig);
>  	clear_user_return_notifier(tsk);
> +	clear_tsk_need_resched(tsk);
>  	stackend = end_of_stack(tsk);
>  	*stackend = STACK_END_MAGIC;	/* for overflow detection */
>  

OK.. have we looked if there's more TIF flags that could do with a
reset?
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