On Fri, Apr 11, 2014 at 08:00:23AM -0700, Andy Lutomirski wrote: > 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.
Right; I forgot to write about that; I was going to look at both ttwu and arch_send_call_function_single_ipi() after this got sorted. While you didn't complain about the remote function call IPI, Venki (while @google) did and this was their reason to look at this. > 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. Today Mike also noted that on very high freq the IPI is actually a lot slower than doing the remote accesses for some weird reason -- previously I've seen the remote wakeups queue a lot of wakeups and have the IPI take too long. So there's definitely something to prod at there. -- 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/
