linux-kernel - Re: [PATCH] kvm: x86: make lapic hrtimer pinned

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Date:	Tue, 5 Apr 2016 08:40:46 -0400
From:	Luiz Capitulino <lcapitulino@...hat.com>
To:	Yang Zhang <yang.zhang.wz@...il.com>
Cc:	Rik van Riel <riel@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, pbonzini@...hat.com,
	rkrcmar@...hat.com, mtosatti@...hat.com, bsd@...hat.com
Subject: Re: [PATCH] kvm: x86: make lapic hrtimer pinned

On Tue, 5 Apr 2016 14:18:01 +0800
Yang Zhang <yang.zhang.wz@...il.com> wrote:

> On 2016/4/5 5:00, Rik van Riel wrote:
> > On Mon, 2016-04-04 at 16:46 -0400, Luiz Capitulino wrote:
> >> When a vCPU runs on a nohz_full core, the hrtimer used by
> >> the lapic emulation code can be migrated to another core.
> >> When this happens, it's possible to observe milisecond
> >> latency when delivering timer IRQs to KVM guests.
> >>
> >> The huge latency is mainly due to the fact that
> >> apic_timer_fn() expects to run during a kvm exit. It
> >> sets KVM_REQ_PENDING_TIMER and let it be handled on kvm
> >> entry. However, if the timer fires on a different core,
> >> we have to wait until the next kvm exit for the guest
> >> to see KVM_REQ_PENDING_TIMER set.
> >>
> >> This problem became visible after commit 9642d18ee. This
> >> commit changed the timer migration code to always attempt
> >> to migrate timers away from nohz_full cores. While it's
> >> discussable if this is correct/desirable (I don't think
> >> it is), it's clear that the lapic emulation code has
> >> a requirement on firing the hrtimer in the same core
> >> where it was started. This is achieved by making the
> >> hrtimer pinned.
> >
> > Given that delivering a timer to a guest seems to
> > involve trapping from the guest to the host, anyway,
> > I don't see a downside to your patch.
> >
> > If that is ever changed (eg. allowing delivery of
> > a timer interrupt to a VCPU without trapping to the
> > host), we may want to revisit this.
> 
> 
> Posted interrupt helps in this case. Currently, KVM doesn't use PI for 
> lapic timer is due to same affinity for lapic timer and VCPU. Now, we 
> can change to use PI for lapic timer. The only concern is what's 
> frequency of timer migration in upstream Linux? If it is frequently, 
> will it bring additional cost?

I can't answer this questions.

> BTW, in what case the migration of timers during VCPU scheduling will fail?

For hrtimers (which is the lapic emulation case), it only succeeds if
the destination core has a hrtimer expiring before the hrtimer being
migrated.

Also, if the hrtimer callback function is already running (that is,
the timer fired already) it's not migrated either. But I _guess_ this
case doesn't affect KVM (and there's no much do about it anyways).