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

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Date:	Fri, 16 Aug 2013 18:26:54 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.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 08/16, Frederic Weisbecker wrote:
>
> On Fri, Aug 16, 2013 at 06:02:01PM +0200, Oleg Nesterov wrote:
> > > +	do {
> > > +		seq = read_seqcount_begin(&ts->sleeptime_seq);
> > > +		if (ts->idle_active && nr_iowait_cpu(cpu) > 0) {
> > > +			ktime_t delta = ktime_sub(now, ts->idle_entrytime);
> > > +			iowait = ktime_add(ts->iowait_sleeptime, delta);
> > > +		} else {
> > > +			iowait = ts->iowait_sleeptime;
> > > +		}
> > > +	} while (read_seqcount_retry(&ts->sleeptime_seq, seq));
> >
> > 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.
>
> Hmm, by the time it decrements nr_iowait, it returned from schedule() and
> so idle had flushed the pending iowait sleeptime.

Suppose a task does io_schedule() on CPU_0, and increments the counter.
This CPU becomes idle and nr_iowait_cpu(0) == 1.

Then this task is woken up, but try_to_wake_up() selects another CPU != 0.

It returns from schedule() and decrements the same counter, it doesn't
do raw_rq/etc again. nr_iowait_cpu(0) becomes 0.

In fact the task can even migrate to another CPU right after raw_rq().

> May be you have some scenario in mind that I'm missing?

Or me...

Oleg.

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