[PATCH net v3] net/mlx5: Use effective affinity mask for IRQ selection

[Fushuai Wang <fushuai.wang@linux.dev>]

From: Fushuai Wang <wangfushuai@baidu.com>

When a sf is created after a CPU has been taken offline, the IRQ pool may
contain IRQs with affinity masks that include the offline CPU. Since only
online CPUs should be considered for IRQ placement, cpumask_subset() check
would fail because the iter_mask contains offline CPUs that are not present
in req_mask, causing sf creation to fail.

This is an example:
  1. When mlx5 driver loads, it initializes the IRQ pools.
     For sf_ctrl_pool with ≤64 sf:
     - xa_num_irqs = {N, N} (There is only one slot)
  2. When the first SF is created:
     - The ctrl IRQ is allocated with mask=cpu_online_mask={0-191}
  2. We take CPU 20 offline
  3. Existing ctl irq still have mask={0-191}
  4. Create a new SF:
     - req_mask={0-19,21-191}
     - iter_mask={0-191}
     - {0-191} is NOT a subset of {0-19,21-191}
     - least_loaded_irq=NULL
  5. Try to allocate a new irq via irq_pool_request_irq()
  6. xa_alloc() fails because the pool is full(There is only one slot)
  7. sf creation fails with error

Use irq_get_effective_affinity_mask() instead, which returns the IRQ's
actual effective affinity that already excludes offline CPUs.

Fixes: 061f5b23588a ("net/mlx5: SF, Use all available cpu for setting cpu affinity")
Suggested-by: Shay Drory <shayd@nvidia.com>
Signed-off-by: Fushuai Wang <wangfushuai@baidu.com>
---
v2->v3: Separate the patchset to two patches, reverse xmas tree coding style fix.
v1->v2: Use mlx5_irq_get_affinity_mask() api

previous discussion:
https://lore.kernel.org/all/20260603072657.10868-1-fushuai.wang@linux.dev/T/#u
https://lore.kernel.org/all/20260604125705.21241-1-fushuai.wang@linux.dev/T/#u

 drivers/net/ethernet/mellanox/mlx5/core/irq_affinity.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/irq_affinity.c b/drivers/net/ethernet/mellanox/mlx5/core/irq_affinity.c
index 994fe83da4be..a0bb8ee44e35 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/irq_affinity.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/irq_affinity.c
@@ -105,9 +105,12 @@ irq_pool_find_least_loaded(struct mlx5_irq_pool *pool, const struct cpumask *req
 
 	lockdep_assert_held(&pool->lock);
 	xa_for_each_range(&pool->irqs, index, iter, start, end) {
-		struct cpumask *iter_mask = mlx5_irq_get_affinity_mask(iter);
 		int iter_refcount = mlx5_irq_read_locked(iter);
+		const struct cpumask *iter_mask;
 
+		iter_mask = irq_get_effective_affinity_mask(mlx5_irq_get_irq(iter));
+		if (!iter_mask)
+			continue;
 		if (!cpumask_subset(iter_mask, req_mask))
 			/* skip IRQs with a mask which is not subset of req_mask */
 			continue;
-- 
2.36.1