Messages in this thread | | | Date | Fri, 16 Aug 2013 18:33:55 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock |
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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.
Oleg.
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