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

From: Marc Zyngier
Date: Wed Mar 18 2020 - 10:04:11 EST


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...