Re: [PATCH][KVM][retry 1] Add support for Pause Filtering to AMD SVM

From: Avi Kivity <avi@redhat.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>,
	Mark Langsdorf <mark.langsdorf@amd.com>,
	Joerg Roedel <joerg.roedel@amd.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH][KVM][retry 1] Add support for Pause Filtering to AMD SVM
Date: Mon, 11 May 2009 18:05:01 +0300	[thread overview]
Message-ID: <4A083E9D.2040003@redhat.com> (raw)
In-Reply-To: <1242052937.11251.275.camel@twins>

Peter Zijlstra wrote:
> On Mon, 2009-05-11 at 17:24 +0300, Avi Kivity wrote:
>
>   
>>> I.e. this is a somewhat poor solution as far as scheduling goes. But 
>>> i'm wondering what the CPU side does. Can REP-NOP really take 
>>> thousands of cycles? If yes, under what circumstances?
>>>   
>>>       
>> The guest is running rep-nop in a loop while trying to acquire a 
>> spinlock.  The hardware detects this (most likely, repeated rep-nop with 
>> the same rip) and exits.  We can program the loop count; obviously if 
>> we're spinning for only a short while it's better to keep spinning while 
>> hoping the lock will be released soon.
>>
>> The idea is to detect that the guest is not making forward progress and 
>> yield.  If I could tell the scheduler, you may charge me a couple of 
>> milliseconds, I promise not to sue, that would be ideal.  Other tasks 
>> can become eligible, hopefully the task holding the spinlock, and by the 
>> time we're scheduled back the long running task will have finished and 
>> released the lock.
>>
>> For newer Linux as a guest we're better off paravirtualizing this, so we 
>> can tell the host which vcpu holds the lock; in this case kvm will want 
>> to say, take a couple milliseconds off my account and transfer it to 
>> this task (so called directed yield).  However there's no reason to 
>> paravirtualize all cpu_relax() calls.
>>     
>
> So we're now officially giving up on (soft) realtime virtualization?
>
>   

Wouldn't realtime guests be in a realtime scheduling class?  That ought 
to ignore this time_yield() (or however it is eventually named).

-- 
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