On Mon, 9 Sep 2019 at 18:56, Waiman Long longman@redhat.com wrote:
On 9/9/19 2:40 AM, Wanpeng Li wrote:
From: Wanpeng Li wanpengli@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 scorevanilla + 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: loobinliu@tencent.com Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Ingo Molnar mingo@kernel.org Cc: Waiman Long longman@redhat.com Cc: Paolo Bonzini pbonzini@redhat.com Cc: Radim Krčmář rkrcmar@redhat.com Cc: loobinliu@tencent.com Cc: stable@vger.kernel.org Fixes: 75437bb304b20 (locking/pvqspinlock: Don't wait if vCPU is preempted) Signed-off-by: Wanpeng Li wanpengli@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:
- Multiple vcpus calling vcpu_is_preempted() repeatedly may cause some
cacheline contention issue depending on how that callback is implemented.
- 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.
pv_wait_again:1:179 pv_wait_early:1:189429 pv_wait_head:1:263 pv_wait_node:1:189429 pv_vcpu_is_preempted:1:45588 =========sleep 5============ pv_wait_again:1:181 pv_wait_early:1:202574 pv_wait_head:1:267 pv_wait_node:1:202590 pv_vcpu_is_preempted:1:46336
The sampling period is 5s, 6% of wait_early events caused by vcpu_is_preempted() being true.
Wanpeng