On Fri, Apr 11, 2014 at 6:42 AM, Peter Zijlstra <pet...@infradead.org> wrote: > A while ago both Mike and Andy complained that we still get pointless wakeup > IPIs, we had a few patches back and forth but eventually more or less agreed > and then nothing... :-) > > So here's a number of patches that implement something near what we left off > with. > > Its only been compile/boot tested on x86_64, I've no actually looked at the > IPI > numbers yet. >
Looks generally good and it should be a nice cleanup. That being said, I think that this addresses once one of the two major issues. While the race you're fixing is more interesting, I think its impact is dwarfed by the fact that ttwu_queue_remote completely ignores polling. (NB: I haven't actually tested this patch set, but I did try to instrument this stuff awhile ago.) To fix this, presumably the wake-from-idle path needs a sched_ttwu_pending call, and ttwu_queue_remote could use resched_task. sched_ttwu_pending could benefit from a straightforward optimization: it doesn't need rq->lock if llist is empty. If you're not planning on trying to fix that, I can try to write up a patch in the next day or two. Even with all of this fixed, what happens when ttwu_queue_remote is called with a task that has lower priority than whatever is currently running on the targeted cpu? I think the result is an IPI that serves very little purpose other than avoiding taking a spinlock in the waking thread. This may be a bad tradeoff. I doubt that this matters for my particular workload, though. --Andy -- 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/
