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

[Raghavendra K T]

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@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().

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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