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

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Message-ID: <20130821083130.GM3258@twins.programming.kicks-ass.net>
Date:	Wed, 21 Aug 2013 10:31:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
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 Tue, Aug 20, 2013 at 08:25:53PM +0200, Oleg Nesterov wrote:
> On 08/20, Peter Zijlstra wrote:
> >
> > On Tue, Aug 20, 2013 at 06:33:12PM +0200, Oleg Nesterov wrote:

> > > +		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-- ?

Yes its the right rq, but the wrong time.

> 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.

No it doesn't. It only does the decrement when the task is woken back
up. Not right after it switches out.

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

I suppose we could, a timeout of MAX_SCHEDULE_TIMEOUT will act like a
regular schedule, but it gets all the overhead of doing
schedule_timeout(). So I don't think its a win.
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