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

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

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> > From: Fushuai Wang <wangfushuai@baidu.com>
> > 
> > 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.
> 

Sure, here are the AI summarized steps:

  1. When mlx5 driver loads, it initializes the IRQ pools.
     For sf_ctrl_pool with ≤64 SFs:
     - 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


> > 
> > 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... :(

Yes, I didn't realize this.

> can you check if irq_get_effective_affinity_mask() will solve the issue?
> 

Yes, I tested that irq_get_effective_affinity_mask can solve the issue.
I will send a v2 shortly.

-- 
Regards,
WANG