linux-kernel - Re: [PATCH] KVM: LAPIC: Periodically revaluate appropriate lapic_timer_advance

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Message-ID: <ba07fb02-9b55-15e4-d240-24da59e09369@redhat.com>
Date:   Wed, 14 Aug 2019 14:50:01 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] KVM: LAPIC: Periodically revaluate appropriate
 lapic_timer_advance_ns

On 12/08/19 11:06, Wanpeng Li wrote:
> On Fri, 9 Aug 2019 at 18:24, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>> On 09/08/19 07:45, Wanpeng Li wrote:
>>> From: Wanpeng Li <wanpengli@...cent.com>
>>>
>>> Even if for realtime CPUs, cache line bounces, frequency scaling, presence
>>> of higher-priority RT tasks, etc can cause different response. These
>>> interferences should be considered and periodically revaluate whether
>>> or not the lapic_timer_advance_ns value is the best, do nothing if it is,
>>> otherwise recaluate again.
>>
>> How much fluctuation do you observe between different runs?
> 
> Sometimes can ~1000 cycles after converting to guest tsc freq.

Hmm, I wonder if we need some kind of continuous smoothing.  Something like

        if (abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_DONE) {
                /* no update for random fluctuations */
		return;
	}

        if (unlikely(timer_advance_ns > 5000))
                timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
        apic->lapic_timer.timer_advance_ns = timer_advance_ns;

and removing all the timer_advance_adjust_done stuff.  What do you think?

Paolo