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?
Oleg.
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