On Mon, Nov 24, 2014 at 03:53:16PM +0800, Shannon Zhao wrote:
> Hi Marc, Christoffer,
>
> On 2014/11/23 4:04, Christoffer Dall wrote:
> > On Wed, Nov 19, 2014 at 06:11:25PM +0800, Shannon Zhao wrote:
> >> When call kvm_vgic_inject_irq to inject interrupt, we can known which
> >> vcpu the interrupt for by the irq_num and the cpuid. So we should just
> >> kick this vcpu to avoid iterating through all.
> >>
> >> Signed-off-by: Shannon Zhao <zhaoshenglong@xxxxxxxxxx>
> >
> > This looks reasonable to me:
> >
> > Reviewed-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
> >
> > But as Marc said, we have to consider the churn by introducing more
> > changes to the vgic (that file is being hammered pretty intensely
> > these days), so if you feel this is an urgent optimization, it would
> > be useful to see some data backing this up.
> >
>
> Today I have a test which measures the cycles about kvm_vgic_inject_irq by PMU.
> I just test the cycles of SPI using virtio-net.
> Test steps:
> 1) start a VM with 8 VCPUs
> 2) In guest bind the irq of virtio to CPU8, host ping VM, get the cycles
>
>
> The test shows:
> Without this patch, the cycles is about 3700(3300-5000), and with this patch, the cycles is about 3000(2500-3200).
> From this test, I think this patch can bring some improvements.
Are these averaged numbers?
>
> The test code like below. As it's almost no difference about vgic_update_irq_state
> between with and without this patch. So just measure the kick's cycles.
>
> int kvm_vgic_inject_irq(struct kvm *kvm, int cpuid, unsigned int irq_num,
> bool level)
> {
> unsigned long cycles_1,cycles_2;
> if (likely(vgic_initialized(kvm)) &&
> vgic_update_irq_pending(kvm, cpuid, irq_num, level)) {
> start_pmu();
> __asm__ __volatile__("MRS %0, PMCCNTR_EL0" : "=r"(cycles_1));
> vgic_kick_vcpus(kvm);
> __asm__ __volatile__("MRS %0, PMCCNTR_EL0" : "=r"(cycles_2));
> }
>
> return 0;
> }
>
> int kvm_vgic_inject_irq(struct kvm *kvm, int cpuid, unsigned int irq_num,
> bool level)
> {
> int vcpu_id;
> unsigned long cycles_a,cycles_b;
> if (likely(vgic_initialized(kvm))) {
> vcpu_id = vgic_update_irq_pending(kvm, cpuid, irq_num, level);
> if (vcpu_id >= 0) {
> start_pmu();
> __asm__ __volatile__("MRS %0, PMCCNTR_EL0" : "=r"(cycles_a));
> /* kick the specified vcpu */
> kvm_vcpu_kick(kvm_get_vcpu(kvm, vcpu_id));
> __asm__ __volatile__("MRS %0, PMCCNTR_EL0" : "=r"(cycles_b));
> }
> }
> return 0;
> }
>
Can you run some IPI-intensive benchmark in your guest and let us know
if you see improvements on that level?
Not trying to be overly-pedantic here (I think your numbers suggest we
should merge this), but if the case you're optimizing doesn't happen
very often, we may not see this on a guest level or overall CPU
utilization level, and it would be very interesting to know.
Thanks!
-Christoffer
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