Re: [PATCH v5 01/14] genirq: Provide new interfaces for affinity hints

On Wed, Jul 21, 2021 at 7:26 AM Nitesh Narayan Lal [off-list ref] wrote:

From: Thomas Gleixner <redacted>

The discussion about removing the side effect of irq_set_affinity_hint() of
actually applying the cpumask (if not NULL) as affinity to the interrupt,
unearthed a few unpleasantries:

  1) The modular perf drivers rely on the current behaviour for the very
     wrong reasons.

  2) While none of the other drivers prevents user space from changing
     the affinity, a cursorily inspection shows that there are at least
     expectations in some drivers.

#1 needs to be cleaned up anyway, so that's not a problem

#2 might result in subtle regressions especially when irqbalanced (which
   nowadays ignores the affinity hint) is disabled.

Provide new interfaces:

  irq_update_affinity_hint()  - Only sets the affinity hint pointer
  irq_set_affinity_and_hint() - Set the pointer and apply the affinity to
                                the interrupt

Make irq_set_affinity_hint() a wrapper around irq_apply_affinity_hint() and
document it to be phased out.

Signed-off-by: Thomas Gleixner <redacted>
Signed-off-by: Nitesh Narayan Lal <redacted>
Link: https://lore.kernel.org/r/20210501021832.743094-1-jesse.brandeburg@intel.com (local)

Reviewed-by: Ming Lei <redacted>