Re: [PATCH v2 5/9] perf/cxl: Keep the overflow interrupt pinned to the managed CPU

[Robin Murphy <robin.murphy@arm.com>]

On 29/07/2026 3:55 pm, Dave Jiang wrote:
> The PMU pins its overflow interrupt to info->on_cpu in the hotplug
> online/offline callbacks, but requests it with only IRQF_SHARED |
> IRQF_NO_THREAD. Without IRQF_NOBALANCING, irqbalance or a userspace
> smp_affinity write can move the interrupt to another CPU. cxl_pmu_irq()
> then runs cxl_pmu_read() there, doing local64_cmpxchg()/local64_add() on
> hwc->prev_count and event->count concurrently with the managing CPU;
> local64_t is only atomic against same-CPU access, so counts get
> corrupted.
> 
> Add IRQF_NOBALANCING so the pinning done in the hotplug callbacks holds,
> matching other uncore-style PMU drivers.
> 
> Fixes: 5d7107c72796 ("perf: CXL Performance Monitoring Unit driver")
> Reported-by: sashiko-bot@kernel.org
> Closes: https://sashiko.dev/#/patchset/20260715191454.459673-1-dave@stgolabs.net?part=1
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> ---
>   drivers/perf/cxl_pmu.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/perf/cxl_pmu.c b/drivers/perf/cxl_pmu.c
> index d1e810601e36..8b89db8f4d68 100644
> --- a/drivers/perf/cxl_pmu.c
> +++ b/drivers/perf/cxl_pmu.c
> @@ -888,7 +888,8 @@ static int cxl_pmu_probe(struct device *dev)
>   	if (!irq_name)
>   		return -ENOMEM;
>   
> -	rc = devm_request_irq(dev, irq, cxl_pmu_irq, IRQF_SHARED | IRQF_NO_THREAD,
> +	rc = devm_request_irq(dev, irq, cxl_pmu_irq,
> +			      IRQF_SHARED | IRQF_NO_THREAD | IRQF_NOBALANCING,

Bah, sorry, now I see I misspoke just now on the other patch - PMUs 
really _shouldn't_ permit shared IRQs, but this one does :(

Thus it's all well and good to prevent userspace changing affinity, but 
it doesn't help _all_ that much if other drivers can still legitimately 
change it behind our backs...

Thanks,
Robin.

>   			      irq_name, info);
>   	if (rc)
>   		return rc;