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

[John Garry]

> +
> +/*
> + * 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 */
> +			if (cpumask_empty(tmpmask))
> +				cpumask_and(tmpmask, cpumask_of_node(node), cpu_online_mask);
> +
> +			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 */
> +		}
> +
> +		/* Try the intersection of the affinity and online masks */
> +		cpumask_and(tmpmask, aff_mask, cpu_online_mask);
> +
> +		/* If that doesn't fly, the online mask is the last resort */
> +		if (cpumask_empty(tmpmask))
> +			cpumask_copy(tmpmask, cpu_online_mask);
> +
> +		cpu = cpumask_pick_least_loaded(d, tmpmask);
> +	} else {


Hi Marc,


> +		cpumask_and(tmpmask, irq_data_get_affinity_mask(d), cpu_online_mask);
> +

Please consider this flow:

- in its_irq_domain_activate()->its_select_cpu(), for a managed 
interrupt we select the target cpu from the interrupt affin mask

- then in its_set_affinity() call for the same interrupt, we may 
needlessly reselect the target cpu. This is because in the 
its_set_affinity()->its_select_cpu() call, we account for that interrupt 
in the load for the current target cpu, and may find a lesser loaded cpu 
in the mask and switch.

For example, from mask 0-5 we select cpu0 initially. Then on the 2nd 
call, we find cpu0 has a greater load (1) then cpu1 (0), and switch to cpu1.

Cheers,
John


> +		/* If we cannot cross sockets, limit the search to that node */
> +		if ((its_dev->its->flags & ITS_FLAGS_WORKAROUND_CAVIUM_23144) &&
> +		    node != NUMA_NO_NODE)
> +			cpumask_and(tmpmask, tmpmask, cpumask_of_node(node));
> +
> +		cpu = cpumask_pick_least_loaded(d, tmpmask);
> +	}
> +out:
> +	free_cpumask_var(tmpmask);
> +
> +	pr_debug("IRQ%d -> %*pbl CPU%d\n", d->irq, cpumask_pr_args(aff_mask), cpu);
> +	return cpu;
> +}