On 11/07/12 13:04, Avi Kivity wrote: > On 07/11/2012 01:17 PM, Christian Borntraeger wrote: >> On 11/07/12 11:06, Avi Kivity wrote: >> [...] >>>> Almost all s390 kernels use diag9c (directed yield to a given guest cpu) for spinlocks, though. >>> >>> Perhaps x86 should copy this. >> >> See arch/s390/lib/spinlock.c >> The basic idea is using several heuristics: >> - loop for a given amount of loops >> - check if the lock holder is currently scheduled by the hypervisor >> (smp_vcpu_scheduled, which uses the sigp sense running instruction) >> Dont know if such thing is available for x86. It must be a lot cheaper >> than a guest exit to be useful > > We could make it available via shared memory, updated using preempt > notifiers. Of course piling on more pv makes this less attractive. > >> - if lock holder is not running and we looped for a while do a directed >> yield to that cpu. >> >>> >>>> So there is no win here, but there are other cases were diag44 is used, e.g. cpu_relax. >>>> I have to double check with others, if these cases are critical, but for now, it seems >>>> that your dummy implementation for s390 is just fine. After all it is a no-op until >>>> we implement something. >>> >>> Does the data structure make sense for you? If so we can move it to >>> common code (and manage it in kvm_vcpu_on_spin()). We can guard it with >>> CONFIG_KVM_HAVE_CPU_RELAX_INTERCEPT or something, so other archs don't >>> have to pay anything. >> >> Ignoring the name, > > What name would you suggest? maybe vcpu_no_progress instead of pause_loop_exited > >> yes the data structure itself seems based on the algorithm >> and not on arch specific things. That should work. If we move that to common >> code then s390 will use that scheme automatically for the cases were we call >> kvm_vcpu_on_spin(). All others archs as well. > > ARM doesn't have an instruction for cpu_relax(), so it can't intercept > it. Given ppc's dislike of overcommit, and the way it implements > cpu_relax() by adjusting hw thread priority, I'm guessing it doesn't > intercept those either, but I'm copying the ppc people in case I'm > wrong. So it's s390 and x86. > >> So this would probably improve guests that uses cpu_relax, for example >> stop_machine_run. I have no measurements, though. > > smp_call_function() too (though that can be converted to directed yield > too). It seems worthwhile. Indeed. For those places where is is possible I would like to see an architecture primitive for directed yield. That could be useful for other places as well (e.g. maybe lib/spinlock_debug.c, which has no yielding at all) Christian -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
