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Message-ID: <20190916185551.GL18871@linux.intel.com>
Date: Mon, 16 Sep 2019 11:55:51 -0700
From: Sean Christopherson <sean.j.christopherson@...el.com>
To: Wanpeng Li <kernellwp@...il.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
Paolo Bonzini <pbonzini@...hat.com>,
Radim Krčmář <rkrcmar@...hat.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v4] KVM: LAPIC: Tune lapic_timer_advance_ns smoothly
On Mon, Sep 16, 2019 at 04:39:59PM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
>
> Filter out drastic fluctuation and random fluctuation, remove
> timer_advance_adjust_done altogether, the adjustment would be
> continuous.
>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
> arch/x86/kvm/lapic.c | 26 ++++++++++++--------------
> arch/x86/kvm/lapic.h | 1 -
> arch/x86/kvm/x86.c | 2 +-
> arch/x86/kvm/x86.h | 1 +
> 4 files changed, 14 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index dbbe478..2585b86 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -69,6 +69,7 @@
> #define LAPIC_TIMER_ADVANCE_ADJUST_INIT 1000
> /* step-by-step approximation to mitigate fluctuation */
> #define LAPIC_TIMER_ADVANCE_ADJUST_STEP 8
> +#define LAPIC_TIMER_ADVANCE_FILTER 5000
>
> static inline int apic_test_vector(int vec, void *bitmap)
> {
> @@ -1482,29 +1483,28 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
> s64 advance_expire_delta)
> {
> struct kvm_lapic *apic = vcpu->arch.apic;
> - u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns;
> - u64 ns;
> + u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns, ns;
Is changing 'ns' to a u32 intentionaly? It's still cast to a u32 in the
calculations, and set from @advance_expire_delta.
> +
> + if (abs(advance_expire_delta) > LAPIC_TIMER_ADVANCE_FILTER ||
Shouldn't LAPIC_TIMER_ADVANCE_FILTER be used in the other "if ... > 5000"
check?
if (unlikely(timer_advance_ns > 5000))
And maybe name it LAPIC_TIMER_ADVANCE_MAX or something?
> + abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_DONE) {
This should be aligned with the other 'abs', e.g.:
if (abs(...) ||
abs(...))
return
> + /* filter out random fluctuations */
If you put the comment above the if statement then you can drop the
parentheses. And if you're going to bother with a comment, maybe be a bit
more explicit? E.g.:
/* Do not adjust for tiny fluctuations or large random spikes. */
if (abs(...) ||
abs(...))
return;
> + return;
> + }
>
> /* too early */
> if (advance_expire_delta < 0) {
> ns = -advance_expire_delta * 1000000ULL;
> do_div(ns, vcpu->arch.virtual_tsc_khz);
> - timer_advance_ns -= min((u32)ns,
> - timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
> + timer_advance_ns -= ns/LAPIC_TIMER_ADVANCE_ADJUST_STEP;
> } else {
> /* too late */
> ns = advance_expire_delta * 1000000ULL;
> do_div(ns, vcpu->arch.virtual_tsc_khz);
> - timer_advance_ns += min((u32)ns,
> - timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
> + timer_advance_ns += ns/LAPIC_TIMER_ADVANCE_ADJUST_STEP;
> }
>
> - if (abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_DONE)
> - apic->lapic_timer.timer_advance_adjust_done = true;
> - if (unlikely(timer_advance_ns > 5000)) {
> + if (unlikely(timer_advance_ns > 5000))
> timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
> - apic->lapic_timer.timer_advance_adjust_done = false;
> - }
> apic->lapic_timer.timer_advance_ns = timer_advance_ns;
> }
>
> @@ -1524,7 +1524,7 @@ static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
> if (guest_tsc < tsc_deadline)
> __wait_lapic_expire(vcpu, tsc_deadline - guest_tsc);
>
> - if (unlikely(!apic->lapic_timer.timer_advance_adjust_done))
> + if (lapic_timer_advance_ns == -1)
Rather than expose 'lapic_timer_advance_ns' from x86.c, what if we add a
'static bool dynamically_adjust_timer_advance __read_mostly;' in lapic.c,
and have that be set in kvm_create_lapic() and checked here? That'd make
this code a little more readable, would make this patch more obvious (it
wasn't immediately clear why lapic_timer_advance_ns was being exposed),
and would reduce the probability of unintentionally writing/corrupting the
module param.
> adjust_lapic_timer_advance(vcpu, apic->lapic_timer.advance_expire_delta);
> }
>
> @@ -2302,10 +2302,8 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, int timer_advance_ns)
> apic->lapic_timer.timer.function = apic_timer_fn;
> if (timer_advance_ns == -1) {
> apic->lapic_timer.timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
> - apic->lapic_timer.timer_advance_adjust_done = false;
> } else {
> apic->lapic_timer.timer_advance_ns = timer_advance_ns;
> - apic->lapic_timer.timer_advance_adjust_done = true;
> }
Parentheses can be dropped (unless this is converted to a local global, as
above).
> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> index 50053d2..2aad7e2 100644
> --- a/arch/x86/kvm/lapic.h
> +++ b/arch/x86/kvm/lapic.h
> @@ -35,7 +35,6 @@ struct kvm_timer {
> s64 advance_expire_delta;
> atomic_t pending; /* accumulated triggered timers */
> bool hv_timer_in_use;
> - bool timer_advance_adjust_done;
> };
>
> struct kvm_lapic {
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 83288ba..bb4574c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -141,7 +141,7 @@ module_param(tsc_tolerance_ppm, uint, S_IRUGO | S_IWUSR);
> * advancement entirely. Any other value is used as-is and disables adaptive
> * tuning, i.e. allows priveleged userspace to set an exact advancement time.
> */
> -static int __read_mostly lapic_timer_advance_ns = -1;
> +int __read_mostly lapic_timer_advance_ns = -1;
> module_param(lapic_timer_advance_ns, int, S_IRUGO | S_IWUSR);
>
> static bool __read_mostly vector_hashing = true;
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index b5274e2..1bf2d82 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -301,6 +301,7 @@ extern unsigned int min_timer_period_us;
>
> extern bool enable_vmware_backdoor;
>
> +extern int lapic_timer_advance_ns;
> extern int pi_inject_timer;
>
> extern struct static_key kvm_no_apic_vcpu;
> --
> 2.7.4
>
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