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