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? > 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. -- error compiling committee.c: too many arguments to function -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
