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

From: Marc Zyngier <[email-masked]>
Date: Wed, 18 Mar 2020 14:04:06 +0000

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/[email-masked]
>> + */
>> +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...

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