Re: [PATCH][retry 2] Add support for Pause Filtering to AMD SVM

Avi Kivity
2009-05-08T18:44:23Z

Mark Langsdorf wrote:
> From 01813db8627e74018c8cec90df7e345839351f23 Mon Sep 17 00:00:00 2001
> From: Mark Langsdorf <mark.langsdorf@amd.com>
> Date: Thu, 7 May 2009 09:44:10 -0500
> Subject: [PATCH] Add support for Pause Filtering to AMD SVM
>

What's the differences wrt retry 1?

> 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.
>

3000.

> 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.
>

Like I mentioned earlier, I don't think schedule() does anything on CFS.

Try sched_yield(), but set /proc/sys/kernel/sched_compat_yield.

> +
> + if (svm_has(SVM_FEATURE_PAUSE_FILTER)) {
> + control->pause_filter_count = 5000;
> + control->intercept |= (1ULL << INTERCEPT_PAUSE);
> + }
> +
>

Here, 5000?

> }
>
> static int svm_vcpu_reset(struct kvm_vcpu *vcpu)
> @@ -2087,6 +2094,15 @@ static int interrupt_window_interception(struct vcpu_svm *svm,
> return 1;
> }
>
> +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;
> +}
> +
>

You don't need to vcpu_put() and vcpu_load(). The scheduler will call
them for you if/when it switches tasks.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.