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

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Date:	Mon, 04 Apr 2016 17:00:23 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Luiz Capitulino <lcapitulino@...hat.com>, kvm@...r.kernel.org
Cc:	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 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.

Until then...

Acked-by: Rik van Riel <riel@...hat.com>

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