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

[Marc Zyngier]

On Thu, 22 Aug 2024 11:59:50 +0100,
Kunkun Jiang <jiangkunkun at huawei.com> wrote:
> 
> 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.

This seems crazy. Why would you build a kernel with something *that*
big, specially considering that you have a lot less than 1k CPUs?

[...]

> > 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?

Given the number of CPUs your system is configured for, there is no
good answer. An on-stack variable is dangerously large, and a per-CPU
cpumask results in 2MB being allocated, which I find insane.

You'll have to pick your own poison and convince Thomas of the
validity of your approach.

	M.

-- 
Without deviation from the norm, progress is not possible.