From 89340d0935c9296c7b8222b6eab30e67cb57ab82 Mon Sep 17 00:00:00 2001

From: Wanpeng Li <wanpengli@tencent.com>

Date: Mon, 9 Sep 2019 09:40:28 +0800

Subject: [PATCH] Revert "locking/pvqspinlock: Don't wait if vCPU is preempted"

Git-commit: 89340d0935c9296c7b8222b6eab30e67cb57ab82

Patch-mainline: v5.4-rc1

References: bsc#1050549

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)

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>

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>

---

 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 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;

 }

 /*

-- 

2.16.4