Re: [patch 0/8] genirq, perf: Cleanup the abuse of irq_set_affinity_hint()

From: Thomas Gleixner
Date: Tue May 18 2021 - 11:51:49 EST

On Tue, May 18 2021 at 11:48, Will Deacon wrote:
> On Tue, May 18, 2021 at 11:17:25AM +0200, Thomas Gleixner wrote:
>> The modular PMU drivers use irq_set_affinity_hint() to set the affinity
>> for the PMU interrupts, which relies on the undocumented side effect that
>> this function actually sets the affinity under the hood.
>> Setting an hint is clearly not a guarantee and for these PMU interrupts an
>> affinity hint, which is supposed to guide userspace for setting affinity,
>> is beyond pointless, because the affinity of these interrupts cannot be
>> modified from user space.
>> Aside of that the error checks are bogus because the only error which is
>> returned from irq_set_affinity_hint() is when there is no irq descriptor
>> for the interrupt number, but not when the affinity set fails. That's on
>> purpose because the hint can point to an offline CPU.
>> Sigh, if people would at least talk if something is missing...
>> Clean up the mess by exposing irq_set_affinity() and converting the drivers
>> over to that.
> This all looks good to me, thanks. Given the number of PMU drivers it
> touches, it may well conflict with driver work for 5.14. If you put the
> IRQ core stuff on a stable branch, then I could pull that into the Arm
> perf tree and stick all the driver changes on top. That also means any
> new drivers that come in can use irq_set_affinity() right away.
> Does that work for you?

Sure. I'll send you a tag to pull ...