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

[Avi Kivity <avi@redhat.com>]

On 07/16/2012 11:25 AM, Raghavendra K T wrote:
> From: Raghavendra K T <raghavendra.kt@linux.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().

> +{
> +	bool eligible;
> +
> +	eligible = !vcpu->ple.cpu_relax_intercepted ||
> +			(vcpu->ple.cpu_relax_intercepted &&
> +			 vcpu->ple.dy_eligible);
> +
> +	if (vcpu->ple.cpu_relax_intercepted)
> +		vcpu->ple.dy_eligible = !vcpu->ple.dy_eligible;

Probably should assign 'true', since the previous value is essentially
random.

> +
> +	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).




-- 
error compiling committee.c: too many arguments to function