Re: [PATCH 2/2] net/mlx5: Only consider online CPUs in affinity subset check

[Shay Drori]

On 03/06/2026 10:26, Fushuai Wang wrote:
> External email: Use caution opening links or attachments
>
>
> From: Fushuai Wang <wangfushuai@xxxxxxxxx>
>
> When an 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.

Thank for the patch!

can you please provide a full example? for simplicity, lets say the SF
pool is of size of 2 IRQs.

> Filter the affinity mask to only include online CPUs before checking if it's
> a subset of the requested mask, 

won't this cause the affinity mask to be empty, which is kind of missing
the point of this API... :(
can you check if irq_get_effective_affinity_mask() will solve the issue?

Thanks

> ensuring SF creation succeeds in this scenario.
>
> Fixes: 061f5b23588a ("net/mlx5: SF, Use all available cpu for setting cpu affinity")
> Signed-off-by: Fushuai Wang <wangfushuai@xxxxxxxxx>
> ---
>   .../net/ethernet/mellanox/mlx5/core/irq_affinity.c | 14 ++++++++++++--
>   1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/irq_affinity.c b/drivers/net/ethernet/mellanox/mlx5/core/irq_affinity.c
> index 994fe83da4be..8c0df240b888 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/irq_affinity.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/irq_affinity.c
> @@ -102,18 +102,26 @@ irq_pool_find_least_loaded(struct mlx5_irq_pool *pool, const struct cpumask *req
>          struct mlx5_irq *iter;
>          int irq_refcount = 0;
>          unsigned long index;
> +       cpumask_var_t tmp;
>
>          lockdep_assert_held(&pool->lock);
> +
> +       if (!alloc_cpumask_var(&tmp, GFP_ATOMIC))
> +               return NULL;
> +
>          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);
>
> -               if (!cpumask_subset(iter_mask, req_mask))
> +               cpumask_and(tmp, iter_mask, cpu_online_mask);
> +               if (!cpumask_subset(tmp, req_mask))
>                          /* skip IRQs with a mask which is not subset of req_mask */
>                          continue;
> -               if (iter_refcount < pool->min_threshold)
> +               if (iter_refcount < pool->min_threshold) {
>                          /* If we found an IRQ with less than min_thres, return it */
> +                       free_cpumask_var(tmp);
>                          return iter;
> +               }
>                  if (!irq || iter_refcount < irq_refcount) {
>                          /* In case we won't find an IRQ with less than min_thres,
>                           * keep a pointer to the least used IRQ
> @@ -122,6 +130,8 @@ irq_pool_find_least_loaded(struct mlx5_irq_pool *pool, const struct cpumask *req
>                          irq = iter;
>                  }
>          }
> +
> +       free_cpumask_var(tmp);
>          return irq;
>   }
>
> --
> 2.36.1