linux-kernel - Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock

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Date:	Tue, 20 Aug 2013 20:25:53 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Fernando Luis Vázquez Cao 
	<fernando_b1@....ntt.co.jp>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock

On 08/20, Peter Zijlstra wrote:
>
> On Tue, Aug 20, 2013 at 06:33:12PM +0200, Oleg Nesterov wrote:
> > --- x/kernel/sched/core.c
> > +++ x/kernel/sched/core.c
> > @@ -2435,6 +2435,9 @@ need_resched:
> >  		rq->curr = next;
> >  		++*switch_count;
> >  
> > +		if (unlikely(prev->in_iowait))
> > +			rq->nr_iowait++;
> > +
> >  		context_switch(rq, prev, next); /* unlocks the rq */
> >  		/*
> >  		 * The context switch have flipped the stack from under us
> > @@ -2442,6 +2445,12 @@ need_resched:
> >  		 * this task called schedule() in the past. prev == current
> >  		 * is still correct, but it can be moved to another cpu/rq.
> >  		 */
> > +		if (unlikely(prev->in_iowait)) {
> > +			raw_spin_lock_irq(&rq->lock);
> > +			rq->nr_iowait--;
> > +			raw_spin_unlock_irq(&rq->lock);
> > +		}
>
> This seems like the wrong place, this is where you return from
> schedule() running another task,

Yes, but prev is current, and rq should be "correct" for
rq->nr_iowait-- ?

This local var should be equal to its value when this task called
context_switch() in the past.

Like any other variable, like "rq = raw_rq()" in io_schedule().

> not where the task you just send to
> sleep wakes up.

sure, but currently io_schedule() does the same.



Btw. Whatever we do, can't we unify io_schedule/io_schedule_timeout?

Oleg.


--- x/kernel/sched/core.c
+++ x/kernel/sched/core.c
@@ -3939,16 +3939,7 @@ EXPORT_SYMBOL_GPL(yield_to);
  */
 void __sched io_schedule(void)
 {
-	struct rq *rq = raw_rq();
-
-	delayacct_blkio_start();
-	atomic_inc(&rq->nr_iowait);
-	blk_flush_plug(current);
-	current->in_iowait = 1;
-	schedule();
-	current->in_iowait = 0;
-	atomic_dec(&rq->nr_iowait);
-	delayacct_blkio_end();
+	io_schedule_timeout(MAX_SCHEDULE_TIMEOUT);
 }
 EXPORT_SYMBOL(io_schedule);
 

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