On Fri, Aug 16, 2013 at 06:33:55PM +0200, Oleg Nesterov wrote: > 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.
My bad, I thought it was doing: inc(this_rq->nr_iowait) schedule() dec(this_rq->nr_iowait) But it actually uses the src CPU all along. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
