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perf/cxl: Don't share the overflow interrupt, and keep it pinned

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The PMU pins its overflow interrupt to info->on_cpu in the hotplug
callbacks, but requests it with only IRQF_SHARED | IRQF_NO_THREAD. Without
IRQF_NOBALANCING, irqbalance or a userspace smp_affinity write can move it
elsewhere, and cxl_pmu_irq() then runs local64_cmpxchg() and local64_add()
on hwc->prev_count and event->count at the same time as the managing CPU.
local64_t is only atomic against same-CPU access, so the counts corrupt.

The flag alone does not fix that while the line is shared. __setup_irq()
only acts on it for the first action on a line, and a co-owner has no
reason to want our affinity - it would keep taking the interrupt wherever
its own points, running our handler on the wrong CPU.

Drop IRQF_SHARED and add IRQF_NOBALANCING. The spec only recommends a
distinct Interrupt Message Number per CPMU instance (CXL r4.0 8.2.7.1.1),
so a device may put several on one vector, or share with the mailbox or
event log. Such a device now loses a PMU rather than silently miscounting -
and only the PMU, since the preceding patch adds the CPMUs after the event
configuration.

Fixes: 5d7107c727 ("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
Reviewed-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://sashiko.dev/#/patchset/20260715191454.459673-1-dave@stgolabs.net?part=1
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Dave Jiang 2026-08-05 08:59:07 -07:00 committed by Will Deacon
parent 28b2b667f4
commit b899fd3487
1 changed files with 9 additions and 2 deletions
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11
drivers/perf/cxl_pmu.c
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@ -896,7 +896,7 @@ static irqreturn_t cxl_pmu_irq(int irq, void *data)
overflowed = readq(base + CXL_PMU_OVERFLOW_REG);
/* Interrupt may be shared, so maybe it isn't ours */
/* Nothing overflowed, so the device did not raise this */
if (!overflowed)
return IRQ_NONE;
@ -999,7 +999,14 @@ 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,
/*
* The handler must run on info->on_cpu, so the interrupt cannot be
* shared - IRQF_NOBALANCING is only honoured for the first action on a
* line, and a co-owner would keep taking the interrupt wherever its own
* affinity points.
*/
rc = devm_request_irq(dev, irq, cxl_pmu_irq,
IRQF_NO_THREAD | IRQF_NOBALANCING,
irq_name, info);
if (rc)
return rc;