There is a downside of halt_poll_ns since poll is still happen for idle VCPU which can waste cpu usage. This patchset add the ability to adjust halt_poll_ns dynamically, grows halt_poll_ns if an interrupt arrives and shrinks halt_poll_ns when idle VCPU is detected.
There are two new kernel parameters for changing the halt_poll_ns: halt_poll_ns_grow and halt_poll_ns_shrink. Test w/ high cpu overcommit ratio, pin vCPUs, and the halt_poll_ns of halt-poll is the default 500000ns, the max halt_poll_ns of dynamic halt-poll is 2ms. Then watch the %C0 in the dump of Powertop tool. The test method is almost from David. +-----------------+----------------+-------------------+ | | | | | w/o halt-poll | w/ halt-poll | dynamic halt-poll | +-----------------+----------------+-------------------+ | | | | | ~0.9% | ~1.8% | ~1.2% | +-----------------+----------------+-------------------+ The always halt-poll will increase ~0.9% cpu usage for idle vCPUs and the dynamic halt-poll drop it to ~0.3% which means that reduce the 67% overhead introduced by always halt-poll. Signed-off-by: Wanpeng Li <wanpeng...@hotmail.com> --- virt/kvm/kvm_main.c | 41 ++++++++++++++++++++++++++++++++++++++++- 1 file changed, 40 insertions(+), 1 deletion(-) diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index c06e57c..d63790d 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -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, 2ms is enough */ +static unsigned int halt_poll_ns = 2000000; 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; + + /* 500us step */ + if (val == 0 && halt_poll_ns_grow) + val = 500000; + 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)) { @@ -1961,6 +1995,11 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu) cur = ktime_get(); out: + if (waited && vcpu->halt_poll_ns > 0) + shrink_halt_poll_ns(vcpu); + else if (vcpu->halt_poll_ns < halt_poll_ns) + grow_halt_poll_ns(vcpu); + trace_kvm_vcpu_wakeup(ktime_to_ns(cur) - ktime_to_ns(start), waited); } EXPORT_SYMBOL_GPL(kvm_vcpu_block); -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html