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* [PATCH] Revert "locking/pvqspinlock: Don't wait if vCPU is preempted"
@ 2019-09-09  1:40 Wanpeng Li
  2019-09-09 10:56 ` Waiman Long
  0 siblings, 1 reply; 8+ messages in thread
From: Wanpeng Li @ 2019-09-09  1:40 UTC (permalink / raw)
  To: linux-kernel, kvm
  Cc: Paolo Bonzini, Radim Krčmář,
	Sean Christopherson, Vitaly Kuznetsov, Wanpeng Li, Jim Mattson,
	Joerg Roedel, Peter Zijlstra, Thomas Gleixner, Ingo Molnar,
	Waiman Long, loobinliu, stable

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                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: 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;
 }
 
 /*
-- 
2.7.4


^ permalink raw reply related	[flat|nested] 8+ messages in thread

end of thread, other threads:[~2019-09-25  3:15 UTC | newest]

Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-09-09  1:40 [PATCH] Revert "locking/pvqspinlock: Don't wait if vCPU is preempted" Wanpeng Li
2019-09-09 10:56 ` Waiman Long
2019-09-09 11:06   ` Paolo Bonzini
2019-09-09 12:16     ` Wanpeng Li
2019-09-10  5:56   ` Wanpeng Li
2019-09-11  4:25     ` Waiman Long
2019-09-11 13:04       ` Paolo Bonzini
2019-09-25  3:15         ` Wanpeng Li

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