From: Paolo Bonzini <pbonzini@redhat.com>
To: Wanpeng Li <wanpeng.li@hotmail.com>
Cc: "Wanpeng Li" <kernellwp@gmail.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
"Radim Krčmář" <rkrcmar@redhat.com>
Subject: Re: [PATCH v3 1/2] KVM: LAPIC: Tune lapic_timer_advance_ns smoothly
Date: Thu, 12 Sep 2019 14:37:46 +0200 [thread overview]
Message-ID: <8054e73d-1e09-0f98-4beb-3caa501f2ac7@redhat.com> (raw)
In-Reply-To: <TY2PR02MB4160421A8C88D96C8BCB971180B00@TY2PR02MB4160.apcprd02.prod.outlook.com>
On 12/09/19 02:34, Wanpeng Li wrote:
>>> - timer_advance_ns -= min((u32)ns,
>>> - timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
>>> + timer_advance_ns -= ns;
Looking more closely, this assignment...
>>> } 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;
... and this one are dead code now. However...
>>> }
>>>
>>> + timer_advance_ns = (apic->lapic_timer.timer_advance_ns *
>>> + (LAPIC_TIMER_ADVANCE_ADJUST_STEP - 1) + advance_expire_delta) /
>>> + LAPIC_TIMER_ADVANCE_ADJUST_STEP;
... you should instead remove this new assignment and just make the
assignments above just
timer_advance -= ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP;
and
timer_advance -= ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP;
In fact this whole last assignment is buggy, since advance_expire_delta
is in TSC units rather than nanoseconds.
>>> if (abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_DONE)
>>> apic->lapic_timer.timer_advance_adjust_done = true;
>>> if (unlikely(timer_advance_ns > 5000)) {
>> This looks great. But instead of patch 2, why not remove
>> timer_advance_adjust_done altogether?
> It can fluctuate w/o stop.
Possibly because of the wrong calculation of timer_advance_ns?
Paolo
next prev parent reply other threads:[~2019-09-12 12:37 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-28 8:19 [PATCH v3 1/2] KVM: LAPIC: Tune lapic_timer_advance_ns smoothly Wanpeng Li
2019-08-28 8:19 ` [PATCH v3 2/2] KVM: LAPIC: Periodically revaluate to get conservative lapic_timer_advance_ns Wanpeng Li
2019-09-11 15:45 ` [PATCH v3 1/2] KVM: LAPIC: Tune lapic_timer_advance_ns smoothly Paolo Bonzini
[not found] ` <TY2PR02MB4160421A8C88D96C8BCB971180B00@TY2PR02MB4160.apcprd02.prod.outlook.com>
2019-09-12 12:37 ` Paolo Bonzini [this message]
2019-09-16 4:02 ` Wanpeng Li
2019-09-16 7:49 ` Paolo Bonzini
2019-09-16 8:09 ` Wanpeng Li
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