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

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Date:	Tue, 5 Apr 2016 17:54:39 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Yang Zhang <yang.zhang.wz@...il.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Luiz Capitulino <lcapitulino@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, pbonzini@...hat.com,
	mtosatti@...hat.com, bsd@...hat.com
Subject: Re: [PATCH] kvm: x86: make lapic hrtimer pinned

2016-04-05 14:18+0800, Yang Zhang:
> On 2016/4/5 5:00, Rik van Riel wrote:
>>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?

It's a scheduler bug if the timer migration frequency would matter. :)
Additional costs arise when the timer and VCPU are on two different
CPUs.  (e.g. if both CPUs are in deep C-state, we wasted one wakeup;
the timer would sometimes needs to send an interrupt.)

Fine tuned KVM could benefit from having the lapic timer backend on a
different physical core, but the general case would need some experience
to decide.

I think that we'd still want to have timer interrupts on the same
physical core if the host didn't have PI, and the fraction of timers
that can be injected without a guest entry is important to decide
whether PI can make the effort worthwhile.

The biggest benefit might come from handling multiple lapic timers in
one host interrupt.