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

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Date:	Tue, 20 Aug 2013 23:55:03 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Fernando Luis Vázquez Cao 
	<fernando_b1@....ntt.co.jp>
Cc:	Oleg Nesterov <oleg@...hat.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>,
	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 Tue, Aug 20, 2013 at 03:21:11PM +0900, Fernando Luis Vázquez Cao wrote:
> (2013年08月17日 01:46), Frederic Weisbecker wrote:
> >On Fri, Aug 16, 2013 at 06:26:54PM +0200, Oleg Nesterov wrote:
> >>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().
> >Ah I see now. So that indeed yet another race.
> 
> I am sorry for chiming in late.
> 
> That precisely the race I described here:
> 
> https://lkml.org/lkml/2013/7/2/3
> 
> I should have been more concise in my explanation. I apologize.

Reading that again, the description was good. It's rather me who didn't read that
not carefully enough :)

> 
> >Should we flush that iowait to the src CPU? But then it means we must handle
> >concurrent updates to iowait_sleeptime, idle_sleeptime from the migration
> >code and from idle enter / exit.
> >
> >So I fear we need a seqlock.
> >
> >Or we can live with that and still account the whole idle time slept until
> >tick_nohz_stop_idle() to iowait if we called tick_nohz_start_idle() with nr_iowait > 0.
> >All we need is just a new field in ts-> that records on which state we entered
> >idle.
> 
> Another approach could be to shadow ->iowait_sleeptime as
> suggested here: https://lkml.org/lkml/2013/7/2/165

Hmm, yeah it seems that it would enforce the monotonicity but the wrong forward jumps
due to bad ordering could still happen.

Oh and I realize you already suggested to account the iowait time on task migration more than
one month ago. Grr I should really sit down before reading emails.
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