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KVM: dynamic halt-polling

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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: David Matlack <dmatlack@google.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
[Simplify the patch. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Wanpeng Li 2015-09-03 22:07:38 +08:00 committed by Paolo Bonzini
parent 19020f8ab8
commit aca6ff29c4
1 changed files with 51 additions and 2 deletions
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53
virt/kvm/kvm_main.c
View file

@ -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);