On 08/21, Peter Zijlstra wrote: > > On Tue, Aug 20, 2013 at 08:25:53PM +0200, Oleg Nesterov wrote: > > On 08/20, Peter Zijlstra wrote: > > > > > > On Tue, Aug 20, 2013 at 06:33:12PM +0200, Oleg Nesterov wrote: > > > > > + if (unlikely(prev->in_iowait)) { > > > > + raw_spin_lock_irq(&rq->lock); > > > > + rq->nr_iowait--; > > > > + raw_spin_unlock_irq(&rq->lock); > > > > + } > > > > > > This seems like the wrong place, this is where you return from > > > schedule() running another task, > > > > Yes, but prev is current, and rq should be "correct" for > > rq->nr_iowait-- ? > > Yes its the right rq, but the wrong time.
Hmm. Just in case, it is not that I think this patch really makes sense, but I'd like to understand why do you think it is wrong. > > This local var should be equal to its value when this task called > > context_switch() in the past. > > > > Like any other variable, like "rq = raw_rq()" in io_schedule(). > > > > > not where the task you just send to > > > sleep wakes up. > > > > sure, but currently io_schedule() does the same. > > No it doesn't. It only does the decrement when the task is woken back > up. Not right after it switches out. But it is not "after it switches out", it is after it switched back. Lets ignore the locking, if (prev->in_iowait) rq->nr_iowait++; context_switch(prev, next); if (prev->in_iowait) rq->nr_iowait--; >From the task_struct's (current's) pov prev/rq are the same, before or after context_switch(). But from the CPU's pov they differ. And ignoring more details on UP the code above is equivalent to if (prev->in_iowait) rq->nr_iowait++; if (next->in_iowait) rq->nr_iowait--; context_switch(prev, next); No? Yes, need_resched()/preemption can trigger more inc/dec's than io_schedule() does, but I don't think this was your concern. > > Btw. Whatever we do, can't we unify io_schedule/io_schedule_timeout? > > I suppose we could, a timeout of MAX_SCHEDULE_TIMEOUT will act like a > regular schedule, but it gets all the overhead of doing > schedule_timeout(). So I don't think its a win. Well, the only overhead is "if(to == MAX_SCHEDULE_TIMEOUT)" at the start. I don't think it makes sense to copy-and-paste the identical code to avoid it. But please ignore, this is really minor and off-topic. Oleg. -- 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/