Commit aca6ff29 authored by Wanpeng Li's avatar Wanpeng Li Committed by Paolo Bonzini
Browse files

KVM: dynamic halt-polling 



There is a downside of always-poll since poll is still happened for idle
vCPUs which can waste cpu usage. This patchset add the ability to adjust
halt_poll_ns dynamically, to grow halt_poll_ns when shot halt is detected,
and to shrink halt_poll_ns when long halt is detected.

There are two new kernel parameters for changing the halt_poll_ns:
halt_poll_ns_grow and halt_poll_ns_shrink.

                        no-poll      always-poll    dynamic-poll
-----------------------------------------------------------------------
Idle (nohz) vCPU %c0     0.15%        0.3%            0.2%
Idle (250HZ) vCPU %c0    1.1%         4.6%~14%        1.2%
TCP_RR latency           34us         27us            26.7us

"Idle (X) vCPU %c0" is the percent of time the physical cpu spent in
c0 over 60 seconds (each vCPU is pinned to a pCPU). (nohz) means the
guest was tickless. (250HZ) means the guest was ticking at 250HZ.

The big win is with ticking operating systems. Running the linux guest
with nohz=off (and HZ=250), we save 3.4%~12.8% CPUs/second and get close
to no-polling overhead levels by using the dynamic-poll. The savings
should be even higher for higher frequency ticks.

Suggested-by: default avatarDavid Matlack <dmatlack@google.com>
Signed-off-by: default avatarWanpeng Li <wanpeng.li@hotmail.com>
[Simplify the patch. - Paolo]
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 19020f8a

virt/kvm/kvm_main.c

+51 −2
Original line number Diff line number Diff line
@@ -66,9 +66,18 @@ 
MODULE_AUTHOR("Qumranet");

MODULE_LICENSE("GPL");



static unsigned int halt_poll_ns;

/* halt polling only reduces halt latency by 5-7 us, 500us is enough */

static unsigned int halt_poll_ns = 500000;

module_param(halt_poll_ns, uint, S_IRUGO | S_IWUSR);



/* Default doubles per-vcpu halt_poll_ns. */

static unsigned int halt_poll_ns_grow = 2;

module_param(halt_poll_ns_grow, int, S_IRUGO);



/* Default resets per-vcpu halt_poll_ns . */

static unsigned int halt_poll_ns_shrink;

module_param(halt_poll_ns_shrink, int, S_IRUGO);



/*

 * Ordering of locks:

 *
@@ -1907,6 +1916,31 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu *vcpu, gfn_t gfn) 
}

EXPORT_SYMBOL_GPL(kvm_vcpu_mark_page_dirty);



static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)

{

	int val = vcpu->halt_poll_ns;



	/* 10us base */

	if (val == 0 && halt_poll_ns_grow)

		val = 10000;

	else

		val *= halt_poll_ns_grow;



	vcpu->halt_poll_ns = val;

}



static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)

{

	int val = vcpu->halt_poll_ns;



	if (halt_poll_ns_shrink == 0)

		val = 0;

	else

		val /= halt_poll_ns_shrink;



	vcpu->halt_poll_ns = val;

}



static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)

{

	if (kvm_arch_vcpu_runnable(vcpu)) {
@@ -1929,6 +1963,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu) 
	ktime_t start, cur;

	DEFINE_WAIT(wait);

	bool waited = false;

	u64 block_ns;



	start = cur = ktime_get();

	if (vcpu->halt_poll_ns) {
@@ -1961,7 +1996,21 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu) 
	cur = ktime_get();



out:

	trace_kvm_vcpu_wakeup(ktime_to_ns(cur) - ktime_to_ns(start), waited);

	block_ns = ktime_to_ns(cur) - ktime_to_ns(start);



	if (halt_poll_ns) {

		if (block_ns <= vcpu->halt_poll_ns)

			;

		/* we had a long block, shrink polling */

		else if (vcpu->halt_poll_ns && block_ns > halt_poll_ns)

			shrink_halt_poll_ns(vcpu);

		/* we had a short halt and our poll time is too small */

		else if (vcpu->halt_poll_ns < halt_poll_ns &&

			block_ns < halt_poll_ns)

			grow_halt_poll_ns(vcpu);

	}



	trace_kvm_vcpu_wakeup(block_ns, waited);

}

EXPORT_SYMBOL_GPL(kvm_vcpu_block);

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