On 9/9/19 2:40 AM, Wanpeng Li wrote: > From: Wanpeng Li <wanpen...@tencent.com> > > This patch reverts commit 75437bb304b20 (locking/pvqspinlock: Don't wait if > vCPU is preempted), we found great regression caused by this commit. > > Xeon Skylake box, 2 sockets, 40 cores, 80 threads, three VMs, each is 80 > vCPUs. > The score of ebizzy -M can reduce from 13000-14000 records/s to 1700-1800 > records/s with this commit. > > Host Guest score > > vanilla + w/o kvm optimizes vanilla 1700-1800 records/s > vanilla + w/o kvm optimizes vanilla + revert 13000-14000 records/s > vanilla + w/ kvm optimizes vanilla 4500-5000 records/s > vanilla + w/ kvm optimizes vanilla + revert 14000-15500 records/s > > Exit from aggressive wait-early mechanism can result in yield premature and > incur extra scheduling latency in over-subscribe scenario. > > kvm optimizes: > [1] commit d73eb57b80b (KVM: Boost vCPUs that are delivering interrupts) > [2] commit 266e85a5ec9 (KVM: X86: Boost queue head vCPU to mitigate lock > waiter preemption) > > Tested-by: loobin...@tencent.com > Cc: Peter Zijlstra <pet...@infradead.org> > Cc: Thomas Gleixner <t...@linutronix.de> > Cc: Ingo Molnar <mi...@kernel.org> > Cc: Waiman Long <long...@redhat.com> > Cc: Paolo Bonzini <pbonz...@redhat.com> > Cc: Radim Krčmář <rkrc...@redhat.com> > Cc: loobin...@tencent.com > Cc: sta...@vger.kernel.org > Fixes: 75437bb304b20 (locking/pvqspinlock: Don't wait if vCPU is preempted) > Signed-off-by: Wanpeng Li <wanpen...@tencent.com> > --- > kernel/locking/qspinlock_paravirt.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/locking/qspinlock_paravirt.h > b/kernel/locking/qspinlock_paravirt.h > index 89bab07..e84d21a 100644 > --- a/kernel/locking/qspinlock_paravirt.h > +++ b/kernel/locking/qspinlock_paravirt.h > @@ -269,7 +269,7 @@ pv_wait_early(struct pv_node *prev, int loop) > if ((loop & PV_PREV_CHECK_MASK) != 0) > return false; > > - return READ_ONCE(prev->state) != vcpu_running || > vcpu_is_preempted(prev->cpu); > + return READ_ONCE(prev->state) != vcpu_running; > } > > /*
There are several possibilities for this performance regression: 1) Multiple vcpus calling vcpu_is_preempted() repeatedly may cause some cacheline contention issue depending on how that callback is implemented. 2) KVM may set the preempt flag for a short period whenver an vmexit happens even if a vmenter is executed shortly after. In this case, we may want to use a more durable vcpu suspend flag that indicates the vcpu won't get a real vcpu back for a longer period of time. Perhaps you can add a lock event counter to count the number of wait_early events caused by vcpu_is_preempted() being true to see if it really cause a lot more wait_early than without the vcpu_is_preempted() call. I have no objection to this, I just want to find out the root cause of it. Cheers, Longman
