Am 26.11.2014 um 10:23 schrieb David Hildenbrand: >> This change is a trade-off. >> PRO: This patch would improve the case of preemption on s390. This is probably a corner case as most distros have preemption off anyway. >> CON: The downside is that kvm_vcpu_yield_to is called also from kvm_vcpu_on_spin. Here we want to avoid the scheduler overhead for a wrong decision. > > Won't most of that part be covered by: > if (!ACCESS_ONCE(vcpu->preempted)) Hmm, right. Checking vcpu->preempted and PF_VCPU might boil down to the same. Would be good if to have to performance regression test, though. > > vcpu->preempted is only set when scheduled out involuntarily. It is cleared > when scheduled in. s390 sets it manually, to speed up waking up a vcpu. > > So when our task is scheduled in (an PF_VCPU is set), this check will already > avoid scheduler overhead in kvm_vcpu_on_spin() or am I missing something? > CC Raghavendra KT. Could be rerun your kernbench/sysbench/ebizzy setup on x86 to see if the patch in this thread causes any regression? If think your commit 7bc7ae25b143"kvm: Iterate over only vcpus that are preempted" might have really made the PF_VCPU check unnecessary CC Michael Mueller, do we still have our yield performance setup handy to check if this patch causes any regression? 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
