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

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Date:	Fri, 16 Aug 2013 18:37:10 +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

2013/8/16 Frederic Weisbecker <fweisbec@...il.com>:
> On Fri, Aug 16, 2013 at 06:02:01PM +0200, Oleg Nesterov wrote:
>> Thanks Frederic!
>>
>> I'll try to read this series carefully later. Not that I think
>> I can help, you certainly understand this much better.
>>
>> Just one question below,
>>
>> On 08/16, Frederic Weisbecker wrote:
>> >
>> > @@ -499,12 +509,15 @@ u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
>> >     if (last_update_time)
>> >             *last_update_time = ktime_to_us(now);
>> >
>> > -   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;
>> > -   }
>> > +   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.
>>
>> 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? I don't
> see the nr_iowait is decreased from the src CPU and increased on the dest CPU.
>
> I don't either see that iowait tasks can't be migrated.

My bad, The decrement happens on the src CPU anyway.
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