Peter Zijlstra <pet...@infradead.org> writes: > On Wed, Jul 09, 2014 at 09:07:53AM +0800, Yuyang Du wrote: >> That is chalenging... Can someone (Peter) grant us a lock of the remote rq? >> :) > > Nope :-).. we got rid of that lock for a good reason. > > Also, this is one area where I feel performance really trumps > correctness, we can fudge the blocked load a little. So the > sched_clock_cpu() difference is a strict upper bound on the > rq_clock_task() difference (and under 'normal' circumstances shouldn't > be much off).
Well, unless IRQ_TIME_ACCOUNTING or such is on, in which case you lose. Or am I misunderstanding the suggestion? Actually the simplest thing would probably be to grab last_update_time (which on 32-bit could be done with the _copy hack) and use that. Then I think the accuracy is only worse than current in that you can lose runnable load as well as blocked load, and that it isn't as easily corrected - currently if the blocked tasks wake up they'll add the correct numbers to runnable_load_avg, even if blocked_load_avg is screwed up and hit zero. This code would have to wait until it stabilized again. > > So we could simply use a timestamps from dequeue and one from enqueue, > and use that. > > As to the remote subtraction, a RMW on another cacheline than the > rq->lock one should be good; esp since we don't actually observe the > per-rq total often (once per tick or so) I think, no? Yeah, it's definitely a different cacheline, and the current code only reads per-ms or on loadbalance migration. > > The thing is, we do not want to disturb scheduling on whatever cpu the > task last ran on if we wake it to another cpu. Taking rq->lock wrecks > that for sure. -- 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/
