On Thu, May 27, 2021 at 6:03 AM Shung-Hsi Yu [off-list ref] wrote:

Hi,

On Fri, May 21, 2021 at 02:03:06PM +0200, Thomas Gleixner wrote:

quoted

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_apply_affinity_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.

Is there recommended way to retrieve the CPU number that the interrupt has
affinity?

Previously a driver (I'm looking at drivers/net/ethernet/amazon/ena) that
uses irq_set_affinity_hint() to spread out IRQ knows the corresponding CPU
number since they're using their own spreading scheme. Now, phasing out
irq_set_affinity_hint(), and thus relying on request_irq() to spread the
load instead, there don't seem to be a easy way to get the CPU number.

For drivers that don't want to rely on request_irq for spreading and want
to force their own affinity mask can use irq_set_affinity() which is an
exported interface now [1] and clearly indicates the purpose of the usage.

As Thomas suggested we are still keeping irq_set_affinity_hint() as a
wrapper until we make appropriate changes in individual drivers that use
this API for different reasons. Please feel free to send out a patch
for this driver once the changes are merged.

[1] https://lkml.org/lkml/2021/5/18/271

-- 
Thanks
Nitesh