blob: 1b27f4045b934800b9850281f88be3ee19e061c8 [file] [log] [blame]
From 217050052dc8c7e3b35a37c6dbcf0adeaaba3c77 Mon Sep 17 00:00:00 2001
From: Wanpeng Li <>
Date: Mon, 9 Sep 2019 09:40:28 +0800
Subject: [PATCH] Revert "locking/pvqspinlock: Don't wait if vCPU is preempted"
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commit 89340d0935c9296c7b8222b6eab30e67cb57ab82 upstream.
This patch reverts commit 75437bb304b20 (locking/pvqspinlock: Don't
wait if vCPU is preempted). A large performance regression was caused
by this commit. on over-subscription scenarios.
The test was run on a Xeon Skylake box, 2 sockets, 40 cores, 80 threads,
with three VMs of 80 vCPUs each. The score of ebizzy -M is reduced from
13000-14000 records/s to 1700-1800 records/s:
Host Guest score
vanilla w/o kvm optimizations upstream 1700-1800 records/s
vanilla w/o kvm optimizations revert 13000-14000 records/s
vanilla w/ kvm optimizations upstream 4500-5000 records/s
vanilla w/ kvm optimizations revert 14000-15500 records/s
Exit from aggressive wait-early mechanism can result in premature yield
and extra scheduling latency.
Actually, only 6% of wait_early events are caused by vcpu_is_preempted()
being true. However, when one vCPU voluntarily releases its vCPU, all
the subsequently waiters in the queue will do the same and the cascading
effect leads to bad performance.
kvm optimizations:
[1] commit d73eb57b80b (KVM: Boost vCPUs that are delivering interrupts)
[2] commit 266e85a5ec9 (KVM: X86: Boost queue head vCPU to mitigate lock waiter preemption)
Cc: Peter Zijlstra <>
Cc: Thomas Gleixner <>
Cc: Ingo Molnar <>
Cc: Waiman Long <>
Cc: Paolo Bonzini <>
Cc: Radim Krčmář <>
Fixes: 75437bb304b20 (locking/pvqspinlock: Don't wait if vCPU is preempted)
Signed-off-by: Wanpeng Li <>
Signed-off-by: Paolo Bonzini <>
Signed-off-by: Paul Gortmaker <>
diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
index 89bab079e7a4..e84d21aa0722 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;