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    SubjectRe: [PATCH v6 2/3] KVM: dynamic halt_poll_ns adjustment
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    On 9/3/15 2:09 AM, David Matlack wrote:
    > On Wed, Sep 2, 2015 at 12:29 AM, Wanpeng Li <wanpeng.li@hotmail.com> 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@google.com>
    >> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
    >> ---
    >> 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.

    Ah, I remember why I keep poll_ns calculation in if(). Actually poll_ns
    has already been accumulated into wait_ns(see how wait_ns is calculated)
    if polling does not succeed. If put poll_ns calculation before if() I
    need to reset variable 'start' to ktime_get().

    Regards,
    Wanpeng Li

    >
    >> + 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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