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    SubjectRe: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock
    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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