[PATCH v3 3/7] alpha/perf: clear PERF_HES_UPTODATE when an event starts counting

[Matt Turner]

alpha_pmu_add() sets PERF_HES_UPTODATE and maybe_change_configuration()
then programs and enables the counter, but nothing clears the flag until
the first alpha_pmu_start(). In that window alpha_pmu_stop() with
PERF_EF_UPDATE skips the count update, because the flag claims the count
is current. Since the next reload rewrites the counter, everything
counted since the event was added is lost.

perf_adjust_freq_unthr_events() does this on the first tick
after an event in frequency mode is scheduled in, which is the default
for perf record, and it then sees no count to adjust the period with.

Clear the flag where the event starts counting. alpha_pmu_add() cannot
do it, since the event is not programmed until the PMU is enabled, and
stopping it before then must not read the counter.

Fixes: a4eaf7f14675 ("perf: Rework the PMU methods")
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Matt Turner <mattst88@xxxxxxxxx>
---
 arch/alpha/kernel/perf_event.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/alpha/kernel/perf_event.c b/arch/alpha/kernel/perf_event.c
index 70840c05d39b..ecec78c355cd 100644
--- a/arch/alpha/kernel/perf_event.c
+++ b/arch/alpha/kernel/perf_event.c
@@ -456,8 +456,11 @@ static void maybe_change_configuration(struct cpu_hw_events *cpuc)
 			cpuc->current_idx[j] = idx;
 		}
 
-		if (!(hwc->state & PERF_HES_STOPPED))
+		if (!(hwc->state & PERF_HES_STOPPED)) {
+			/* The counter is live, so hwc->prev_count will go stale. */
+			hwc->state &= ~PERF_HES_UPTODATE;
 			cpuc->idx_mask |= (1<<cpuc->current_idx[j]);
+		}
 	}
 	cpuc->config = cpuc->event[0]->hw.config_base;
 }

-- 
2.55.0