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

On Wed, 29 Jul 2026 07:55:51 -0700
Dave Jiang <[email protected]> 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: [email protected]
> Closes: https://sashiko.dev/#/patchset/[email protected]?part=1
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Dave Jiang <[email protected]>
Reviewed-by: Jonathan Cameron <[email protected]>

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