Re: [PATCH v3 0/2] irqchip/gic-v3-its: Balance LPI affinity across CPUs

[John Garry]

Hi Marc,

> But I would also like to report some other unexpected behaviour for 
> managed interrupts in this series - I'll reply directly to the specific 
> patch for that.

So I made this change:

diff --git a/drivers/irqchip/irq-gic-v3-its.c 
b/drivers/irqchip/irq-gic-v3-its.c
index 9199fb53c75c..ebbfc8d44d35 100644
--- a/drivers/irqchip/irq-gic-v3-its.c
+++ b/drivers/irqchip/irq-gic-v3-its.c
@@ -1539,6 +1539,8 @@ static int its_set_affinity(struct irq_data *d, 
const struct cpumask *mask_val,
        if (irqd_is_forwarded_to_vcpu(d))
                return -EINVAL;

+       its_dec_lpi_count(d, its_dev->event_map.col_map[id]);
+
        if (!force)
                cpu = its_select_cpu(d, mask_val);
        else
@@ -1549,14 +1551,14 @@ static int its_set_affinity(struct irq_data *d, 
const struct cpumask *mask_val,

        /* don't set the affinity when the target cpu is same as 
current one */
        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));
        }

+       its_inc_lpi_count(d, cpu);
+
        return IRQ_SET_MASK_OK_DONE;
 }

Results look ok:
	nvme.use_threaded_interrupts=1	=0*
Before	950K IOPs			1000K IOPs
After	1100K IOPs			1150K IOPs

* as mentioned before, this is quite unstable and causes lockups. JFYI, 
there was an attempt to fix this:

https://lore.kernel.org/linux-nvme/20191209175622.1964-1-kbusch@xxxxxxxxxx/

Thanks,
John