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

Avi Kivity
2009-05-07T15:31:24Z

(copying Ingo)

Mark Langsdorf wrote:
> commit 01813db8627e74018c8cec90df7e345839351f23
> Author: root <root@xendinar01.amd.com>
> Date: Thu May 7 09:44:10 2009 -0500
>
> New AMD processors will support the Pause Filter Feature.
> This feature creates a new field in the VMCB called Pause
> Filter Count. If Pause Filter Count is greater than 0 and
> intercepting PAUSEs is enabled, the processor will increment
> an internal counter when a PAUSE instruction occurs instead
> of intercepting. When the internal counter reaches the
> Pause Filter Count value, a PAUSE intercept will occur.
>
> This feature can be used to detect contended spinlocks,
> especially when the lock holding VCPU is not scheduled.
> Rescheduling another VCPU prevents the VCPU seeking the
> lock from wasting its quantum by spinning idly.
>
> Experimental results show that most spinlocks are held
> for less than 1000 PAUSE cycles or more than a few
> thousand. Default the Pause Filter Counter to 3000 to
> detect the contended spinlocks.
>
> Processor support for this feature is indicated by a CPUID
> bit.
>
> On a 24 core system running 4 guests each with 16 VCPUs,
> this patch improved overall performance of each guest's
> 32 job kernbench by approximately 1%. Further performance
> improvement may be possible with a more sophisticated
> yield algorithm.
>
> -Mark Langsdorf
> Operating System Research Center
> AMD
>
> Signed-off-by: Mark Langsdorf <mark.langsdorf@amd.com>
>

(please use git format-patch rather than git show, and set up user.name
and user.email properly)

>
> svm->nested_vmcb = 0;
> svm->vcpu.arch.hflags = HF_GIF_MASK;
> +
> + if (svm_has(SVM_FEATURE_PAUSE_FILTER)) {
> + control->pause_filter_count = 5000;
> + control->intercept |= (1ULL << INTERCEPT_PAUSE);
> + }
> +
> }

3000 or 5000?

>
> +static int pause_interception(struct vcpu_svm *svm, struct kvm_run *kvm_run)
> +{
> + /* Simple yield */
> + vcpu_put(&svm->vcpu);
> + schedule();
> + vcpu_load(&svm->vcpu);
> + return 1;
> +

Ingo, will this do anything under CFS, or will CFS note that nothing has
changed in the accounting and reschedule us immediately?

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