Re: [bug report] GICv4.1: multiple vpus execute vgic_v4_load at the same time will greatly increase the time consumption

[Kunkun Jiang]

Hi Marc,

On 2024/8/22 16:26, Marc Zyngier wrote:
> > > > According to analysis, this problem is due to the execution of vgic_v4_load.
> > > > vcpu_load or kvm_sched_in
> > > >       kvm_arch_vcpu_load
> > > >       ...
> > > >           vgic_v4_load
> > > >               irq_set_affinity
> > > >               ...
> > > >                   irq_do_set_affinity
> > > >                       raw_spin_lock(&tmp_mask_lock)
> > > >                       chip->irq_set_affinity
> > > >                       ...
> > > >                         its_vpe_set_affinity
> > > >
> > > > The tmp_mask_lock is the key. This is a global lock. I don't quite
> > > > understand
> > > > why tmp_mask_lock is needed here. I think there are two possible
> > > > solutions here:
> > > > 1. Remove this tmp_mask_lock
> > >
> > > Maybe you could have a look at 33de0aa4bae98 (and 11ea68f553e24)? It
> > > would allow you to understand the nature of the problem.
> > >
> > > This can probably be replaced with a per-CPU cpumask, which would
> > > avoid the locking, but potentially result in a larger memory usage.
> >
> > Thanks, I will try it.
>
> A simple alternative would be this:
>
> diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
> index dd53298ef1a5..0d11b74af38c 100644
> --- a/kernel/irq/manage.c
> +++ b/kernel/irq/manage.c
> @@ -224,15 +224,12 @@ int irq_do_set_affinity(struct irq_data *data, const struct cpumask *mask,
>   	struct irq_desc *desc = irq_data_to_desc(data);
>   	struct irq_chip *chip = irq_data_get_irq_chip(data);
>   	const struct cpumask  *prog_mask;
> +	struct cpumask tmp_mask = {};
>   	int ret;
>   
> -	static DEFINE_RAW_SPINLOCK(tmp_mask_lock);
> -	static struct cpumask tmp_mask;
> -
>   	if (!chip || !chip->irq_set_affinity)
>   		return -EINVAL;
>   
> -	raw_spin_lock(&tmp_mask_lock);
>   	/*
>   	 * If this is a managed interrupt and housekeeping is enabled on
>   	 * it check whether the requested affinity mask intersects with
> @@ -280,8 +277,6 @@ int irq_do_set_affinity(struct irq_data *data, const struct cpumask *mask,
>   	else
>   		ret = -EINVAL;
>   
> -	raw_spin_unlock(&tmp_mask_lock);
> -
>   	switch (ret) {
>   	case IRQ_SET_MASK_OK:
>   	case IRQ_SET_MASK_OK_DONE:
>
> but that will eat a significant portion of your stack if your kernel is
> configured for a large number of CPUs.

Currently CONFIG_NR_CPUS=4096,each `struct cpumask` occupies 512 bytes.

> > > > 2. Modify the gicv4 driver,do not perfrom VMOVP via
> > > > irq_set_affinity.
> > >
> > > Sure. You could also not use KVM at all if don't care about interrupts
> > > being delivered to your VM. We do not send a VMOVP just for fun. We
> > > send it because your vcpu has moved to a different CPU, and the ITS
> > > needs to know about that.
> >
> > When a vcpu is moved to a different CPU, of course VMOVP has to be sent.
> > I mean is it possible to call its_vpe_set_affinity() to send VMOVP by
> > other means (instead of by calling the irq_set_affinity() API). So we
> > can bypass this tmp_mask_lock.
>
> The whole point of this infrastructure is that the VPE doorbell is the
> control point for the VPE. If the VPE moves, then the change of
> affinity *must* be done using irq_set_affinity(). All the locking is
> constructed around that. Please read the abundant documentation that
> exists in both the GIC code and KVM describing why this is done like
> that.

OK. Thank you for your guidance.

> > > You seem to be misunderstanding the use case for GICv4: a partitioned
> > > system, without any over-subscription, no vcpu migration between CPUs.
> > > If that's not your setup, then GICv4 will always be a net loss
> > > compared to SW injection with GICv3 (additional HW interaction,
> > > doorbell interrupts).
> >
> > Thanks for the explanation. The key to the problem is not vcpu migration
> > between CPUs. The key point is that many vcpus execute vgic_v4_load() at
> > the same time. Even if it is not migrated to another CPU, there may be a
> > large number of vcpus executing vgic_v4_load() at the same time. For
> > example, the service running in VMs has a large number of MMIO accesses
> > and need to return to userspace for emulation. Due to the competition of
> > tmp_mask_lock, performance will deteriorate.
>
> That's only a symptom. And that doesn't affect only pathological VM
> workloads, but all interrupts being moved around for any reason.

Yes.

> > When the target CPU is the same CPU as the last run, there seems to be
> > no need to call irq_set_affinity() in this case. I did a test and it was
> > indeed able to alleviate the problem described above.
>
> The premise is that irq_set_affinity() should be cheap when there
> isn't much to do, and you are papering over the problem.
>
> > I feel it might be better to remove tmp_mask_lock or call
> > its_vpe_set_affinity() in another way. So I mentioned these two ideas
> > above.
>
> The removal of this global lock is the only option in my opinion.
> Either the cpumask becomes a stack variable, or it becomes a static
> per-CPU variable. Both have drawbacks, but they are not a bottleneck
> anymore.

I also prefer to remove the global lock. Which variable do you think is
better?

Thanks,
Kunkun Jiang