linux-kernel - Re: [PATCH RFC V4 3/3] kvm: Choose better candidate for directed yield

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Date:	Mon, 16 Jul 2012 23:19:12 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	S390 <linux-s390@...r.kernel.org>,
	Carsten Otte <cotte@...ibm.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	KVM <kvm@...r.kernel.org>, chegu vinod <chegu_vinod@...com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, X86 <x86@...nel.org>,
	Gleb Natapov <gleb@...hat.com>, linux390@...ibm.com,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Joerg Roedel <joerg.roedel@....com>
Subject: Re: [PATCH RFC V4 3/3] kvm: Choose better candidate for directed
 yield

On 07/16/2012 03:37 PM, Avi Kivity wrote:
> On 07/16/2012 11:25 AM, Raghavendra K T wrote:
>> From: Raghavendra K T<raghavendra.kt@...ux.vnet.ibm.com>
>>
>> Currently, on a large vcpu guests, there is a high probability of
>> yielding to the same vcpu who had recently done a pause-loop exit or
>> cpu relax intercepted. Such a yield can lead to the vcpu spinning
>> again and hence degrade the performance.
>>
>> The patchset keeps track of the pause loop exit/cpu relax interception
>> and gives chance to a vcpu which:
>>   (a) Has not done pause loop exit or cpu relax intercepted at all
>>       (probably he is preempted lock-holder)
>>   (b) Was skipped in last iteration because it did pause loop exit or
>>       cpu relax intercepted, and probably has become eligible now
>>       (next eligible lock holder)
>>
>
>>
>> +#ifdef CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT
>> +/*
>> + * Helper that checks whether a VCPU is eligible for directed yield.
>> + * Most eligible candidate to yield is decided by following heuristics:
>> + *
>> + *  (a) VCPU which has not done pl-exit or cpu relax intercepted recently
>> + *  (preempted lock holder), indicated by @cpu_relax_intercepted.
>> + *  Set at the beiginning and cleared at the end of interception/PLE handler.
>> + *
>> + *  (b) VCPU which has done pl-exit/ cpu relax intercepted but did not get
>> + *  chance last time (mostly it has become eligible now since we have probably
>> + *  yielded to lockholder in last iteration. This is done by toggling
>> + *  @dy_eligible each time a VCPU checked for eligibility.)
>> + *
>> + *  Yielding to a recently pl-exited/cpu relax intercepted VCPU before yielding
>> + *  to preempted lock-holder could result in wrong VCPU selection and CPU
>> + *  burning. Giving priority for a potential lock-holder increases lock
>> + *  progress.
>> + */
>> +bool kvm_vcpu_check_and_update_eligible(struct kvm_vcpu *vcpu)
>
> Predicates' names should give a hint as to what true and false returns
> mean.  For example vcpu_eligible_for_directed_yield().
>

I agree regarding the Predicate name. My confusion was it was
doing more than that (flipping eligible flag).
So, I  ll go with kvm_vcpu_eligible_for_directed_yield()

>> +{
[...]
>> +	return eligible;
>> +}
>
> You're accessing another vcpu's data structures without any locking.
> This is probably okay since we're not basing any life or death decisions
> on this, but a comment would be good to explain to readers that this has
> been considered and is okay (and why).
>
>

True and agree. What we doing here is not worth of locking overhead.
will try to explain more on that.

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