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

On Wed, 29 Jul 2026, 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.
>

Reviewed-by: Davidlohr Bueso <[email protected]>

>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]>
>---
> 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;
>-- 
>2.55.0
>