On Wed, Sep 2, 2015 at 12:29 AM, Wanpeng Li <wanpeng.li@xxxxxxxxxxx> wrote:
> There is a downside of always-poll since poll is still happened for idle
> vCPUs which can waste cpu usage. This patch adds 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@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
> virt/kvm/kvm_main.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 58 insertions(+), 3 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index c06e57c..3cff02f 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, 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 poll_ns = 0, wait_ns = 0, block_ns = 0;
>
> start = cur = ktime_get();
> if (vcpu->halt_poll_ns) {
> @@ -1941,10 +1976,15 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> */
> if (kvm_vcpu_check_block(vcpu) < 0) {
> ++vcpu->stat.halt_successful_poll;
> - goto out;
> + break;
> }
> cur = ktime_get();
> } while (single_task_running() && ktime_before(cur, stop));
> +
> + if (ktime_before(cur, stop)) {
You can't use 'cur' to tell if the interrupt arrived. single_task_running()
can break us out of the loop before 'stop'.
> + poll_ns = ktime_to_ns(cur) - ktime_to_ns(start);
Put this line before the if(). block_ns should always include the time
spent polling; even if polling does not succeed.
> + goto out;
> + }
> }
>
> for (;;) {
> @@ -1959,9 +1999,24 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>
> finish_wait(&vcpu->wq, &wait);
> cur = ktime_get();
> + wait_ns = ktime_to_ns(cur) - ktime_to_ns(start);
>
> out:
> - trace_kvm_vcpu_wakeup(ktime_to_ns(cur) - ktime_to_ns(start), waited);
> + block_ns = poll_ns + wait_ns;
> +
> + if (halt_poll_ns) {
If you want, you can leave this if() out and save some indentation.
> + 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);
>
> --
> 1.9.1
>
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