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

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Date:	Fri, 16 Aug 2013 18:49:27 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock

On Fri, Aug 16, 2013 at 06:33:55PM +0200, Oleg Nesterov wrote:
> On 08/16, Frederic Weisbecker wrote:
> >
> > On Fri, Aug 16, 2013 at 06:02:01PM +0200, Oleg Nesterov wrote:
> > >
> > > Unless I missread this patch, this is still racy a bit.
> > >
> > > Suppose it is called on CPU_0 and cpu == 1. Suppose that
> > > ts->idle_active == T and nr_iowait_cpu(cpu) == 1.
> > >
> > > So we return iowait_sleeptime + delta.
> > >
> > > Suppose that we call get_cpu_iowait_time_us() again. By this time
> > > the task which incremented ->nr_iowait can be woken up on another
> > > CPU, and it can do atomic_dec(rq->nr_iowait). So the next time
> > > we return iowait_sleeptime, and this is not monotonic again.
> > >
> > > No?
> >
> > OTOH, io_schedule() does:
> >
> >       atomic_inc(&rq->nr_iowait);
> >       schedule();
> >       atomic_dec(&rq->nr_iowait);
> >
> > How do we handle that when the task is migrated after it goes to sleep?
> 
> or even before it goes to sleep. This is what I meant.
> 
> > I don't either see that iowait tasks can't be migrated.
> 
> But probably this is fine? This is just the non-precise accounting.
> 
> But otoh, I agree. The whole idea about per-cpu nr_iowait looks a
> bit strange.


My bad, I thought it was doing:

   inc(this_rq->nr_iowait)
   schedule()
   dec(this_rq->nr_iowait)

But it actually uses the src CPU all along.
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