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

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]>
---
 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