Re: [PATCH v3 2/2] irqchip/gic-v3-its: Balance initial LPI affinity across CPUs

[Marc Zyngier]

On 2020-03-18 12:22, John Garry wrote:
> I may have an idea about this:
> irq 196, cpu list 0-31, effective list 82
>
> Just going back to comment on the code:
>
> > +/*
> > + * As suggested by Thomas Gleixner in:
> > + * https://lore.kernel.org/r/87h80q2aoc.fsf@xxxxxxxxxxxxxxxxxxxxxxx
> > + */
> > +static int its_select_cpu(struct irq_data *d,
> > +			  const struct cpumask *aff_mask)
> > +{
> > +	struct its_device *its_dev = irq_data_get_irq_chip_data(d);
> > +	cpumask_var_t tmpmask;
> > +	int cpu, node;
> > +
> > +	if (!alloc_cpumask_var(&tmpmask, GFP_KERNEL))
> > +		return -ENOMEM;
> > +
> > +	node = its_dev->its->numa_node;
> > +
> > +	if (!irqd_affinity_is_managed(d)) {
> > +		/* First try the NUMA node */
> > +		if (node != NUMA_NO_NODE) {
> > +			/*
> > +			 * Try the intersection of the affinity mask and the
> > +			 * node mask (and the online mask, just to be safe).
> > +			 */
> > +			cpumask_and(tmpmask, cpumask_of_node(node), aff_mask);
> > +			cpumask_and(tmpmask, tmpmask, cpu_online_mask);
> > +
> > +			/* If that doesn't work, try the nodemask itself */
>
> So if tmpmsk is empty...

Which means the proposed affinity mask isn't part of the node mask the 
first place.
Why did we get such an affinity the first place?

> > +			if (cpumask_empty(tmpmask))
> > +				cpumask_and(tmpmask, cpumask_of_node(node), cpu_online_mask);
>
>  now the tmpmask may have no intersection with the aff_mask...

But it has the mask for CPUs that are best suited for this interrupt, 
right?
If I understand the topology of your machine, it has an ITS per 64 CPUs, 
and
this device is connected to the ITS that serves the second socket.

> > +
> > +			cpu = cpumask_pick_least_loaded(d, tmpmask);
> > +			if (cpu < nr_cpu_ids)
> > +				goto out;
> > +
> > +			/* If we can't cross sockets, give up */
> > +			if ((its_dev->its->flags & ITS_FLAGS_WORKAROUND_CAVIUM_23144))
> > +				goto out;
> > +
> > +			/* If the above failed, expand the search */
> > +		}
>
> SNIP
>
> > +out:
> > +	free_cpumask_var(tmpmask);
> > +
> > +	pr_debug("IRQ%d -> %*pbl CPU%d\n", d->irq, 
> > cpumask_pr_args(aff_mask), cpu);
> > +	return cpu;
> > +}
> > +
> >   static int its_set_affinity(struct irq_data *d, const struct cpumask 
> > *mask_val,
> >   			    bool force)
> >   {
> > -	unsigned int cpu;
> > -	const struct cpumask *cpu_mask = cpu_online_mask;
> >   	struct its_device *its_dev = irq_data_get_irq_chip_data(d);
> >   	struct its_collection *target_col;
> >   	u32 id = its_get_event_id(d);
> > +	int cpu;
> >     	/* A forwarded interrupt should use irq_set_vcpu_affinity */
> >   	if (irqd_is_forwarded_to_vcpu(d))
> >   		return -EINVAL;
> >   -       /* lpi cannot be routed to a redistributor that is on a 
> > foreign node */
> > -	if (its_dev->its->flags & ITS_FLAGS_WORKAROUND_CAVIUM_23144) {
> > -		if (its_dev->its->numa_node >= 0) {
> > -			cpu_mask = cpumask_of_node(its_dev->its->numa_node);
> > -			if (!cpumask_intersects(mask_val, cpu_mask))
> > -				return -EINVAL;
> > -		}
> > -	}
> > -
> > -	cpu = cpumask_any_and(mask_val, cpu_mask);
> > +	if (!force)
> > +		cpu = its_select_cpu(d, mask_val);
> > +	else
> > +		cpu = cpumask_pick_least_loaded(d, mask_val);
> >   -	if (cpu >= nr_cpu_ids)
> > +	if (cpu < 0 || cpu >= nr_cpu_ids)
> >   		return -EINVAL;
>
> Annotate missing code:
>
> 	if (cpu < 0 || cpu >= nr_cpu_ids)
> 		return -EINVAL;
>
> 	if (cpu != its_dev->event_map.col_map[id]) {
> 		its_inc_lpi_count(d, cpu);
> 		its_dec_lpi_count(d, its_dev->event_map.col_map[id]);
> 		target_col = &its_dev->its->collections[cpu];
> 		its_send_movi(its_dev, target_col, id);
> 		its_dev->event_map.col_map[id] = cpu;
> 		irq_data_update_effective_affinity(d, cpumask_of(cpu));
> 	}
>
> So cpu may not be a member of mask_val. Hence the inconsistency of the
> affinity list and effective affinity. We could just drop the AND of
> the ITS node mask in its_select_cpu().

That would be a departure from the algorithm Thomas proposed, which made
a lot of sense in my opinion. What its_select_cpu() does in this case is
probably the best that can be achieved from a latency perspective,
as it keeps the interrupt local to the socket that generated it.

What I wonder is how we end-up with this silly aff_mask the first place.

> Anyway, I don't think that this should stop us testing.

Agreed.

        M.
--
Jazz is not dead. It just smells funny...