Messages in this thread | | | Date | Fri, 16 Aug 2013 18:02:01 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock |
| |
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.
| |