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    SubjectRe: [PATCH][KVM][retry 2] Add support for Pause Filtering to AMD SVM
    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.



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