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

[John Garry]

On 19/01/2020 19:05, Marc Zyngier wrote:> When mapping a LPI, the ITS 
driver picks the first possible> affinity, which is in most cases CPU0, 
assuming that if> that's not suitable, someone will come and set the 
affinity> to something more interesting.> > It apparently isn't the 
case, and people complain of poor> performance when many interrupts are 
glued to the same CPU.> So let's place the interrupts by finding the 
"least loaded"> CPU (that is, the one that has the fewer LPIs mapped to 
it).> So called 'managed' interrupts are an interesting case where> the 
affinity is actually dictated by the kernel itself, and> we should honor 
this.> > Reported-by: John Garry <john.garry@xxxxxxxxxx>> Link: 
https://lore.kernel.org/r/1575642904-58295-1-git-send-email-john.garry@xxxxxxxxxx> 
Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
So I tested on NVMe, and it showed a good performance boost - maybe ~20% 
for 2x NVMe disks on my D06. But a couple of comments below:

Thanks,
John

> ---
>   drivers/irqchip/irq-gic-v3-its.c | 120 +++++++++++++++++++++++--------
>   1 file changed, 92 insertions(+), 28 deletions(-)
>
> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> index e05673bcd52b..ec50cc1b11a3 100644
> --- a/drivers/irqchip/irq-gic-v3-its.c
> +++ b/drivers/irqchip/irq-gic-v3-its.c
> @@ -177,6 +177,8 @@ static DEFINE_IDA(its_vpeid_ida);
>   #define gic_data_rdist_rd_base()	(gic_data_rdist()->rd_base)
>   #define gic_data_rdist_vlpi_base()	(gic_data_rdist_rd_base() + SZ_128K)
>   
> +static DEFINE_PER_CPU(atomic_t, cpu_lpi_count);
> +
>   static u16 get_its_list(struct its_vm *vm)
>   {
>   	struct its_node *its;
> @@ -1287,42 +1289,96 @@ static void its_unmask_irq(struct irq_data *d)
>   	lpi_update_config(d, 0, LPI_PROP_ENABLED);
>   }
>   
> +static int its_pick_target_cpu(const struct cpumask *cpu_mask)
> +{
> +	unsigned int cpu = nr_cpu_ids, tmp;
> +	int count = S32_MAX;
> +
> +	/*
> +	 * If we're only picking one of the online CPUs, just pick the
> +	 * first one (there are drivers that depend on this behaviour).
> +	 * At some point, we'll have to weed them out.
> +	 */
> +	if (cpu_mask == cpu_online_mask)

should this use cpumask_equal()?

> +		return cpumask_first(cpu_mask);
> +
> +	for_each_cpu(tmp, cpu_mask) {
> +		int this_count = per_cpu(cpu_lpi_count, tmp).counter;
> +		if (this_count < count) {
> +			cpu = tmp;
> +		        count = this_count;
> +		}
> +	}
> +
> +	return cpu;
> +}
> +
> +static void its_compute_affinity(struct irq_data *d,
> +				 const struct cpumask *requested,
> +				 struct cpumask *computed)
> +{
> +	struct its_device *its_dev = irq_data_get_irq_chip_data(d);
> +
> +	cpumask_and(computed, requested, cpu_online_mask);
> +
> +	/* LPI cannot be routed to a redistributor that is on a foreign node */
> +	if ((its_dev->its->flags & ITS_FLAGS_WORKAROUND_CAVIUM_23144) &&
> +	    its_dev->its->numa_node >= 0)
> +		cpumask_and(computed, computed,
> +			    cpumask_of_node(its_dev->its->numa_node));
> +}
> +
>   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;
> +	int ret = IRQ_SET_MASK_OK_DONE;
>   	u32 id = its_get_event_id(d);
> +	cpumask_var_t tmpmask;
> +	struct cpumask *mask;
>   
>   	/* 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;
> -		}
> +	if (!force) {
> +		if (!alloc_cpumask_var(&tmpmask, GFP_KERNEL))
> +			return -ENOMEM;
> +
> +		mask = tmpmask;
> +		its_compute_affinity(d, mask_val, mask);
> +	} else	{
> +		mask = (struct cpumask *)mask_val;
>   	}
>   
> -	cpu = cpumask_any_and(mask_val, cpu_mask);
> +	if (cpumask_empty(mask)) {
> +		ret = -EINVAL;
> +		goto out;
> +	}
>   
> -	if (cpu >= nr_cpu_ids)
> -		return -EINVAL;
> +	if (!cpumask_test_cpu(its_dev->event_map.col_map[id], mask)) {
> +		struct its_collection *target_col;
> +		int cpu;
> +
> +		cpu = its_pick_target_cpu(mask);
> +		if (cpu >= nr_cpu_ids) {
> +			ret = -EINVAL;
> +			goto out;
> +		}
>   
> -	/* don't set the affinity when the target cpu is same as current one */
> -	if (cpu != its_dev->event_map.col_map[id]) {
> +		atomic_inc(per_cpu_ptr(&cpu_lpi_count, cpu));
> +		atomic_dec(per_cpu_ptr(&cpu_lpi_count,
> +				       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));
>   	}
>   
> -	return IRQ_SET_MASK_OK_DONE;
> +out:
> +	if (!force)
> +		free_cpumask_var(tmpmask);
> +	return ret;
>   }
>   
>   static u64 its_irq_get_msi_base(struct its_device *its_dev)
> @@ -2773,28 +2829,34 @@ static int its_irq_domain_activate(struct irq_domain *domain,
>   {
>   	struct its_device *its_dev = irq_data_get_irq_chip_data(d);
>   	u32 event = its_get_event_id(d);
> -	const struct cpumask *cpu_mask = cpu_online_mask;
> -	int cpu;
> +	int ret = 0, cpu = nr_cpu_ids;
> +	const struct cpumask *reqmask;
> +	cpumask_var_t mask;
>   
> -	/* get the cpu_mask of local node */
> -	if (its_dev->its->numa_node >= 0)
> -		cpu_mask = cpumask_of_node(its_dev->its->numa_node);
> +	if (irqd_affinity_is_managed(d))
> +		reqmask = irq_data_get_affinity_mask(d);
> +	else
> +		reqmask = cpu_online_mask;
>   
> -	/* Bind the LPI to the first possible CPU */
> -	cpu = cpumask_first_and(cpu_mask, cpu_online_mask);
> -	if (cpu >= nr_cpu_ids) {
> -		if (its_dev->its->flags & ITS_FLAGS_WORKAROUND_CAVIUM_23144)
> -			return -EINVAL;
> +	if (!alloc_cpumask_var(&mask, GFP_KERNEL))
> +		return -ENOMEM;
>   
> -		cpu = cpumask_first(cpu_online_mask);
> +	its_compute_affinity(d, reqmask, mask);
> +	cpu = its_pick_target_cpu(mask);
> +	if (cpu >= nr_cpu_ids) {
> +		ret = -EINVAL;
> +		goto out;
>   	}
>   
> +	atomic_inc(per_cpu_ptr(&cpu_lpi_count, cpu));

I wonder if we should only consider managed interrupts in this accounting?

So cpu0 is effectively going to be excluded from the balancing, as it 
will have so many lpis targeted.

And, for the others, even if we balance all the LPIs, won't irqbalance 
(if running, obviously) can come along and fiddle with these non-managed 
interrupt affinities anyway?

>   	its_dev->event_map.col_map[event] = cpu;
>   	irq_data_update_effective_affinity(d, cpumask_of(cpu));
>   
>   	/* Map the GIC IRQ and event to the device */
>   	its_send_mapti(its_dev, d->hwirq, event);
> -	return 0;
> +out:
> +	free_cpumask_var(mask);
> +	return ret;
>   }
>   
>   static void its_irq_domain_deactivate(struct irq_domain *domain,
> @@ -2803,6 +2865,8 @@ static void its_irq_domain_deactivate(struct irq_domain *domain,
>   	struct its_device *its_dev = irq_data_get_irq_chip_data(d);
>   	u32 event = its_get_event_id(d);
>   
> +	atomic_dec(per_cpu_ptr(&cpu_lpi_count,
> +			       its_dev->event_map.col_map[event]));
>   	/* Stop the delivery of interrupts */
>   	its_send_discard(its_dev, event);
>   }