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. David -- 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
