linux-kernel - Re: [PATCH v4 0/5] KVM: LAPIC: Optimize timer latency further

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Date:   Wed, 22 May 2019 16:51:52 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v4 0/5] KVM: LAPIC: Optimize timer latency further

On Mon, 20 May 2019 at 16:18, Wanpeng Li <kernellwp@...il.com> wrote:
>
> Advance lapic timer tries to hidden the hypervisor overhead between the
> host emulated timer fires and the guest awares the timer is fired. However,
> it just hidden the time between apic_timer_fn/handle_preemption_timer ->
> wait_lapic_expire, instead of the real position of vmentry which is
> mentioned in the orignial commit d0659d946be0 ("KVM: x86: add option to
> advance tscdeadline hrtimer expiration"). There is 700+ cpu cycles between
> the end of wait_lapic_expire and before world switch on my haswell desktop.
>
> This patchset tries to narrow the last gap(wait_lapic_expire -> world switch),
> it takes the real overhead time between apic_timer_fn/handle_preemption_timer
> and before world switch into consideration when adaptively tuning timer
> advancement. The patchset can reduce 40% latency (~1600+ cycles to ~1000+
> cycles on a haswell desktop) for kvm-unit-tests/tscdeadline_latency when
> testing busy waits.

Testing on a Skylake Server, w/ nohz=off, idle=poll in the guest.
Reduces average cyclictest latency from 3us to 2us.

Regards,
Wanpeng Li