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: 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>
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1 changed files with 51 additions and 2 deletions
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@ -66,9 +66,18 @@
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MODULE_AUTHOR("Qumranet");
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MODULE_AUTHOR("Qumranet");
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MODULE_LICENSE("GPL");
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MODULE_LICENSE("GPL");
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static unsigned int halt_poll_ns;
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/* halt polling only reduces halt latency by 5-7 us, 500us is enough */
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static unsigned int halt_poll_ns = 500000;
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module_param(halt_poll_ns, uint, S_IRUGO | S_IWUSR);
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module_param(halt_poll_ns, uint, S_IRUGO | S_IWUSR);
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/* Default doubles per-vcpu halt_poll_ns. */
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static unsigned int halt_poll_ns_grow = 2;
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module_param(halt_poll_ns_grow, int, S_IRUGO);
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/* Default resets per-vcpu halt_poll_ns . */
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static unsigned int halt_poll_ns_shrink;
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module_param(halt_poll_ns_shrink, int, S_IRUGO);
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/*
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/*
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* Ordering of locks:
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* Ordering of locks:
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*
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*
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@ -1907,6 +1916,31 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu *vcpu, gfn_t gfn)
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}
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}
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EXPORT_SYMBOL_GPL(kvm_vcpu_mark_page_dirty);
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EXPORT_SYMBOL_GPL(kvm_vcpu_mark_page_dirty);
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static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
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{
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int val = vcpu->halt_poll_ns;
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/* 10us base */
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if (val == 0 && halt_poll_ns_grow)
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val = 10000;
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else
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val *= halt_poll_ns_grow;
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vcpu->halt_poll_ns = val;
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}
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static void shrink_halt_poll_ns(struct kvm_vcpu *vcpu)
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{
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int val = vcpu->halt_poll_ns;
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if (halt_poll_ns_shrink == 0)
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val = 0;
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else
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val /= halt_poll_ns_shrink;
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vcpu->halt_poll_ns = val;
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}
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static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
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static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
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{
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{
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if (kvm_arch_vcpu_runnable(vcpu)) {
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if (kvm_arch_vcpu_runnable(vcpu)) {
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@ -1929,6 +1963,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
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ktime_t start, cur;
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ktime_t start, cur;
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DEFINE_WAIT(wait);
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DEFINE_WAIT(wait);
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bool waited = false;
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bool waited = false;
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u64 block_ns;
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start = cur = ktime_get();
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start = cur = ktime_get();
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if (vcpu->halt_poll_ns) {
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if (vcpu->halt_poll_ns) {
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@ -1961,7 +1996,21 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
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cur = ktime_get();
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cur = ktime_get();
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out:
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out:
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trace_kvm_vcpu_wakeup(ktime_to_ns(cur) - ktime_to_ns(start), waited);
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block_ns = ktime_to_ns(cur) - ktime_to_ns(start);
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if (halt_poll_ns) {
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if (block_ns <= vcpu->halt_poll_ns)
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;
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/* we had a long block, shrink polling */
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else if (vcpu->halt_poll_ns && block_ns > halt_poll_ns)
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shrink_halt_poll_ns(vcpu);
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/* we had a short halt and our poll time is too small */
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else if (vcpu->halt_poll_ns < halt_poll_ns &&
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block_ns < halt_poll_ns)
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grow_halt_poll_ns(vcpu);
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}
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trace_kvm_vcpu_wakeup(block_ns, waited);
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}
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}
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EXPORT_SYMBOL_GPL(kvm_vcpu_block);
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EXPORT_SYMBOL_GPL(kvm_vcpu_block);
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