From 3b8a5df6c4dc6df2ab17d099fb157032f80bdca2 Mon Sep 17 00:00:00 2001 From: Wanpeng Li Date: Tue, 9 Oct 2018 09:02:08 +0800 Subject: KVM: LAPIC: Tune lapic_timer_advance_ns automatically MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit In cloud environment, lapic_timer_advance_ns is needed to be tuned for every CPU generations, and every host kernel versions(the kvm-unit-tests/tscdeadline_latency.flat is 5700 cycles for upstream kernel and 9600 cycles for our 3.10 product kernel, both preemption_timer=N, Skylake server). This patch adds the capability to automatically tune lapic_timer_advance_ns step by step, the initial value is 1000ns as 'commit d0659d946be0 ("KVM: x86: add option to advance tscdeadline hrtimer expiration")' recommended, it will be reduced when it is too early, and increased when it is too late. The guest_tsc and tsc_deadline are hard to equal, so we assume we are done when the delta is within a small scope e.g. 100 cycles. This patch reduces latency (kvm-unit-tests/tscdeadline_latency, busy waits, preemption_timer enabled) from ~2600 cyles to ~1200 cyles on our Skylake server. Cc: Paolo Bonzini Cc: Radim Krčmář Cc: Liran Alon Signed-off-by: Wanpeng Li Signed-off-by: Paolo Bonzini --- arch/x86/kvm/lapic.c | 25 ++++++++++++++++++++++++- 1 file changed, 24 insertions(+), 1 deletion(-) (limited to 'arch/x86/kvm/lapic.c') diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c index fbb0e6df121b..91ffb63397f5 100644 --- a/arch/x86/kvm/lapic.c +++ b/arch/x86/kvm/lapic.c @@ -70,6 +70,11 @@ #define APIC_BROADCAST 0xFF #define X2APIC_BROADCAST 0xFFFFFFFFul +static bool lapic_timer_advance_adjust_done = false; +#define LAPIC_TIMER_ADVANCE_ADJUST_DONE 100 +/* step-by-step approximation to mitigate fluctuation */ +#define LAPIC_TIMER_ADVANCE_ADJUST_STEP 8 + static inline int apic_test_vector(int vec, void *bitmap) { return test_bit(VEC_POS(vec), (bitmap) + REG_POS(vec)); @@ -1472,7 +1477,7 @@ static bool lapic_timer_int_injected(struct kvm_vcpu *vcpu) void wait_lapic_expire(struct kvm_vcpu *vcpu) { struct kvm_lapic *apic = vcpu->arch.apic; - u64 guest_tsc, tsc_deadline; + u64 guest_tsc, tsc_deadline, ns; if (!lapic_in_kernel(vcpu)) return; @@ -1492,6 +1497,24 @@ void wait_lapic_expire(struct kvm_vcpu *vcpu) if (guest_tsc < tsc_deadline) __delay(min(tsc_deadline - guest_tsc, nsec_to_cycles(vcpu, lapic_timer_advance_ns))); + + if (!lapic_timer_advance_adjust_done) { + /* too early */ + if (guest_tsc < tsc_deadline) { + ns = (tsc_deadline - guest_tsc) * 1000000ULL; + do_div(ns, vcpu->arch.virtual_tsc_khz); + lapic_timer_advance_ns -= min((unsigned int)ns, + lapic_timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP); + } else { + /* too late */ + ns = (guest_tsc - tsc_deadline) * 1000000ULL; + do_div(ns, vcpu->arch.virtual_tsc_khz); + lapic_timer_advance_ns += min((unsigned int)ns, + lapic_timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP); + } + if (abs(guest_tsc - tsc_deadline) < LAPIC_TIMER_ADVANCE_ADJUST_DONE) + lapic_timer_advance_adjust_done = true; + } } static void start_sw_tscdeadline(struct kvm_lapic *apic) -- cgit v1.2.3