Re: [PATCH RFC 2/2] kvm: set affinity hint for assigned device msi

From: Michael S. Tsirkin
Date: Mon Oct 17 2011 - 09:31:23 EST

On Mon, Oct 17, 2011 at 08:58:59AM -0200, Marcelo Tosatti wrote:
> On Sun, Oct 16, 2011 at 03:12:23PM +0200, Michael S. Tsirkin wrote:
> > On Thu, Oct 13, 2011 at 11:54:50AM -0300, Marcelo Tosatti wrote:
> > > On Tue, Oct 11, 2011 at 08:38:28PM +0200, Michael S. Tsirkin wrote:
> > > > To forward an interrupt to a vcpu that runs on
> > > > a host cpu different from the current one,
> > > > we need an ipi which likely will cost us as much
> > > > as delivering the interrupt directly to that cpu would.
> > > >
> > > > Set irq affinity hint to point there, irq balancer
> > > > can then take this into accound and balance
> > > > interrupts accordingly.
> > > >
> > > > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > > ---
> > > > virt/kvm/assigned-dev.c | 8 +++++---
> > > > virt/kvm/irq_comm.c | 17 ++++++++++++++++-
> > > > 2 files changed, 21 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/virt/kvm/assigned-dev.c b/virt/kvm/assigned-dev.c
> > > > index f89f138..b579777 100644
> > > > --- a/virt/kvm/assigned-dev.c
> > > > +++ b/virt/kvm/assigned-dev.c
> > > > @@ -142,9 +142,11 @@ static void deassign_host_irq(struct kvm *kvm,
> > > > for (i = 0; i < assigned_dev->entries_nr; i++)
> > > > disable_irq(assigned_dev->host_msix_entries[i].vector);
> > > >
> > > > - for (i = 0; i < assigned_dev->entries_nr; i++)
> > > > - free_irq(assigned_dev->host_msix_entries[i].vector,
> > > > - (void *)assigned_dev);
> > > > + for (i = 0; i < assigned_dev->entries_nr; i++) {
> > > > + u32 vector = assigned_dev->host_msix_entries[i].vector;
> > > > + irq_set_affinity_hint(vector, NULL);
> > > > + free_irq(vector, (void *)assigned_dev);
> > > > + }
> > > >
> > > > assigned_dev->entries_nr = 0;
> > > > kfree(assigned_dev->host_msix_entries);
> > > > diff --git a/virt/kvm/irq_comm.c b/virt/kvm/irq_comm.c
> > > > index ac8b629..68b1f7c 100644
> > > > --- a/virt/kvm/irq_comm.c
> > > > +++ b/virt/kvm/irq_comm.c
> > > > @@ -22,6 +22,7 @@
> > > >
> > > > #include <linux/kvm_host.h>
> > > > #include <linux/slab.h>
> > > > +#include <linux/interrupt.h>
> > > > #include <trace/events/kvm.h>
> > > >
> > > > #include <asm/msidef.h>
> > > > @@ -80,6 +81,17 @@ inline static bool kvm_is_dm_lowest_prio(struct kvm_lapic_irq *irq)
> > > > #endif
> > > > }
> > > >
> > > > +static void kvm_vcpu_host_irq_hint(struct kvm_vcpu *vcpu, int host_irq)
> > > > +{
> > > > + const struct cpumask *mask;
> > > > + /* raw_smp_processor_id() is ok here: if we get preempted we can get a
> > > > + * wrong value but we don't mind much. */
> > > > + if (host_irq >= 0 && unlikely(vcpu->cpu != raw_smp_processor_id())) {
> > > > + mask = get_cpu_mask(vcpu->cpu);
> > > > + irq_set_affinity_hint(host_irq, mask);
> > > > + }
> > > > +}
> > >
> > > Unsure about the internals of irq_set_affinity_hint, but AFAICS its
> > > exported so that irqbalance in userspace can make a decision.
> >
> > Yes. Pls note at the moment there's no hint so irqbalance
> > will likely try to move the irq away from vcpu if that
> > is doing a lot of work. My patch tries to correct that.
> >
> > > If that is the case, then irqbalance update rate should be high enough
> > > to catch up with a vcpu migrating betweens cpus (which initially does
> > > not appear a sensible arrangement).
> >
> > At least for pinned vcpus, that's almost sure to be the case :)
>
> What i mean is that the frequency of a vcpu migrating between cpus
> might be higher than what irqbalance can cope with.
>
> > > The decision to have the host interrupt follow the vcpu seems a good
> > > one, given that it saves an IPI and is potentially more cache friendly
> > > overall.
> >
> > > And AFAICS its more intelligent for the device assignment case than
> > > anything irqbalance can come up with
> >
> > Do you just propose overwriting affinity set by userspace then?
>
> Yes.
>
> > My concern would be to avoid breaking setups some users have,
> > with carefully manually optimized affinity for vcpus and device irqs.
>
> They can disable automatic in-kernel affinity.

This still means code needs to be changed ...
Anyway, what's the interface for that?

> >
> > > (note it depends on how the APIC is
> > > configured, your patch ignores that).
> >
> > Could you clarify please? What is meant by 'it' in 'it depends'?
>
> "It" means the target vcpu selection. It depends on how the guest
> APIC is programmed.
>
> > Which APIC - host or guest - do you mean, and what are possible APIC
> > configurations to consider?
>
> Guest APIC. Guest APIC programmed with round robin would break the
> static assignment on your patch.

For round robin we might just want to disable this
automatic affinity?

> Configurations to consider, all common ones used for assigned devices?

I mean, besides round robin, any other modes that
have an issue? Interrupts can also be multicast,
I think, but we probably don't care what happens
to affinity then, as msi interrupts are probably never
broadcast ...

>
> > > > int kvm_irq_delivery_to_apic(struct kvm *kvm, struct kvm_lapic *src,
> > > > struct kvm_lapic_irq *irq, int host_irq)
> > > > {
> > > > @@ -102,6 +114,7 @@ int kvm_irq_delivery_to_apic(struct kvm *kvm, struct kvm_lapic *src,
> > > > if (r < 0)
> > > > r = 0;
> > > > r += kvm_apic_set_irq(vcpu, irq);
> > > > + kvm_vcpu_host_irq_hint(vcpu, host_irq);
> > > > } else if (kvm_lapic_enabled(vcpu)) {
> > > > if (!lowest)
> > > > lowest = vcpu;
> > > > @@ -110,8 +123,10 @@ int kvm_irq_delivery_to_apic(struct kvm *kvm, struct kvm_lapic *src,
> > > > }
> > > > }
> > > >
> > > > - if (lowest)
> > > > + if (lowest) {
> > > > r = kvm_apic_set_irq(lowest, irq);
> > > > + kvm_vcpu_host_irq_hint(vcpu, host_irq);
> > > > + }
> > > >
> > > > return r;
> > > > }
> > > > --
> > > > 1.7.5.53.gc233e
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