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.
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