On Thu, Sep 05, 2013 at 03:18:14PM +0200, Heiko Carstens wrote: > d472d9d9 "lockref: Relax in cmpxchg loop" added a cpu_relax() call to the > CMPXCHG_LOOP() macro. However to me it seems to be wrong since it is very > likely that the next round will succeed (or the loop will be left). > Even worse: cpu_relax() is very expensive on s390, since it means yield > "my virtual cpu to the hypervisor". So we are talking of several 1000 cycles. > > In fact some measurements show the bad impact of the cpu_relax() call on > s390 using Linus' test case that "stats()" like mad: > > Without converting s390 to lockref: > Total loops: 81236173 > > After converting s390 to lockref: > Total loops: 31896802 > > After converting s390 to lockref but with removed cpu_relax() call: > Total loops: 86242190
All of those should have been "converting s390 to ARCH_USE_CMPXCHG_LOCKREF" instead of "to lockref" of course .. ;) -- 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/