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

[Waiman Long <longman@redhat.com>]

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                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;
>  }
>  
>  /*

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