linux-kernel - Re: [PATCH v2] KVM: halt-polling: poll if emulated lapic timer will fire soon

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Date:	Thu, 19 May 2016 15:57:37 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	David Matlack <dmatlack@...gle.com>,
	Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [PATCH v2] KVM: halt-polling: poll if emulated lapic timer will
 fire soon



On 19/05/2016 15:27, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> If an emulated lapic timer will fire soon(in the scope of 10us the
> base of dynamic halt-polling, lower-end of message passing workload
> latency TCP_RR's poll time < 10us) we can treat it as a short halt,
> and poll to wait it fire, the fire callback apic_timer_fn() will set
> KVM_REQ_PENDING_TIMER, and this flag will be check during busy poll.
> This can avoid context switch overhead and the latency which we wake
> up vCPU.

Would this work too and be simpler?

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 4fd482fb9260..8d42f5304d94 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1964,16 +1964,12 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 
 	old = val = vcpu->halt_poll_ns;
 	grow = READ_ONCE(halt_poll_ns_grow);
-	/* 10us base */
-	if (val == 0 && grow)
-		val = 10000;
-	else
-		val *= grow;
+	val *= grow;
 
 	if (val > halt_poll_ns)
 		val = halt_poll_ns;
 
-	vcpu->halt_poll_ns = val;
+	vcpu->halt_poll_ns = max(10000u, val);
 	trace_kvm_halt_poll_ns_grow(vcpu->vcpu_id, val, old);
 }
 
@@ -1988,7 +1984,7 @@ static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
 	else
 		val /= shrink;
 
-	vcpu->halt_poll_ns = val;
+	vcpu->halt_poll_ns = max(10000u, val);
 	trace_kvm_halt_poll_ns_shrink(vcpu->vcpu_id, val, old);
 }
 

(Plus moving 10000 into a module parameter?)  Can you measure higher CPU
utilization than with your patch?  David, what do you think?

Thanks,

Paolo