On Fri, 16 Sep 2016 11:30:58 +0000 David Laight <David.Laight@xxxxxxxxxx> wrote: > From: Nicholas Piggin > > Sent: 16 September 2016 09:58 > > Implementing busy wait loops with cpu_relax() in callers poses > > some difficulties for powerpc. > > > > First, we want to put our SMT thread into a low priority mode for the > > duration of the loop, but then return to normal priority after exiting > > the loop. Dependong on the CPU design, 'HMT_low() ; HMT_medium();' as > > cpu_relax() does may have HMT_medium take effect before HMT_low made > > any (or much) difference. > > > > Second, it can be beneficial for some implementations to spin on the > > exit condition with a statically predicted-not-taken branch (i.e., > > always predict the loop will exit). > > > > This is a quick RFC with a couple of users converted to see what > > people think. I don't use a C branch with hints, because we don't want > > the compiler moving the loop body out of line, which makes it a bit > > messy unfortunately. If there's a better way to do it, I'm all ears. > > I think it will still all go wrong if the conditional isn't trivial. > In particular if the condition contains || or && it is likely to > have a branch - which could invert the loop. I don't know that it will. Yes, if we have exit condition that requires more branches in order to be computed then we lose our nice property of never taking a branch miss on loop exit. But we still avoid *this* branch miss, and still prevent multiple iterations of the wait loop being speculatively executed concurrently when there's no work to be done. And C doesn't know about the loop, so it can't do any transformation except to compute the final condition. Or have I missed something? Thanks, Nick -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
