Re: [PATCH v19 02/11] perf: arm_pmu: Don't disable counter in armpmu_add()
From: Anshuman Khandual <hidden>
Date: 2025-02-03 06:04:32
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On 2/3/25 06:12, Rob Herring (Arm) wrote:
From: Mark Rutland <mark.rutland@arm.com> Currently armpmu_add() tries to handle a newly-allocated counter having a stale associated event, but this should not be possible, and if this
A stale associated event ? Does that mean hw_events->events[idx] still points to a valid event even though counter idx has already been freed up and allocated to a new event.
were to happen the current mitigation is insufficient and potentially expensive. It would be better to warn if we encounter the impossible case.
Makes sense.
Calls to pmu::add() and pmu::del() are serialized by the core perf code, and armpmu_del() clears the relevant slot in pmu_hw_events::events[] before clearing the bit in pmu_hw_events::used_mask such that the counter can be reallocated. Thus when armpmu_add() allocates a counter index from pmu_hw_events::used_mask, it should not be possible to observe a stale even in pmu_hw_events::events[] unless either pmu_hw_events::used_mask or pmu_hw_events::events[] have been corrupted. If this were to happen, we'd end up with two events with the same event->hw.idx, which would clash with each other during reprogramming, deletion, etc, and produce bogus results. Add a WARN_ON_ONCE() for this case so that we can detect if this ever occurs in practice.
Agreed.
That possiblity aside, there's no need to call arm_pmu::disable(event)
s/possiblity/possibility
quoted hunk ↗ jump to hunk
for the new event. The PMU reset code initialises the counter in a disabled state, and armpmu_del() will disable the counter before it can be reused. Remove the redundant disable. Signed-off-by: Mark Rutland <mark.rutland@arm.com> Signed-off-by: Rob Herring (Arm) <robh@kernel.org> --- drivers/perf/arm_pmu.c | 8 +++----- 1 file changed, 3 insertions(+), 5 deletions(-)diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c index 398cce3d76fc..2f33e69a8caf 100644 --- a/drivers/perf/arm_pmu.c +++ b/drivers/perf/arm_pmu.c@@ -342,12 +342,10 @@ armpmu_add(struct perf_event *event, int flags) if (idx < 0) return idx; - /* - * If there is an event in the counter we are going to use then make - * sure it is disabled. - */ + /* The newly-allocated counter should be empty */
Should this comment also include what happens when two events some how end up using the same 'event->hw.idx' as mentioned in the commit message, just to make things clearer.
+ WARN_ON_ONCE(hw_events->events[idx]); + event->hw.idx = idx; - armpmu->disable(event); hw_events->events[idx] = event; hwc->state = PERF_HES_STOPPED | PERF_HES_UPTODATE;
Otherwise LGTM. Reviewed-by: Anshuman Khandual <redacted>