linux-kernel - Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

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Message-ID: <5474D39F.4010206@redhat.com>
Date:	Tue, 25 Nov 2014 14:08:15 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>, linux-kernel@...r.kernel.org
CC:	Luiz Capitulino <lcapitulino@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, kvm@...r.kernel.org
Subject: Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

On 11/25/2014 01:57 PM, Rik van Riel wrote:
> On 11/25/2014 12:21 PM, Marcelo Tosatti wrote:
>> The problem:
>>
>> On -RT, an emulated LAPIC timer instances has the following path:
>>
>> 1) hard interrupt
>> 2) ksoftirqd is scheduled
>> 3) ksoftirqd wakes up vcpu thread
>> 4) vcpu thread is scheduled
>>
>> This extra context switch introduces unnecessary latency in the 
>> LAPIC path for a KVM guest.
>>
>> The solution:
>>
>> Allow waking up vcpu thread from hardirq context,
>> thus avoiding the need for ksoftirqd to be scheduled.
>>
>> Normal waitqueues make use of spinlocks, which on -RT 
>> are sleepable locks. Therefore, waking up a waitqueue 
>> waiter involves locking a sleeping lock, which 
>> is not allowed from hard interrupt context.
>>
> 
> What are the consequences for the realtime kernel of
> making waitqueue traversal non-preemptible?
> 
> Is waitqueue traversal something that ever takes so
> long we need to care about it being non-preemptible?

I answered my own question.

This patch only changes the kvm vcpu waitqueue,
which should only ever have the vcpu thread itself
waiting on it. In other words, it is a wait "queue"
of just one entry long, and the latency of traversing
it will be absolutely minimal.

Unless someone can think of a better way to implement
this patch, it gets my:

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

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