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Subject[PATCH 09/12] perf/x86-ibs: Extend hw period that triggers overflow
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If the last hw period is too short we might hit the irq handler which
biases the results. Thus try to have a max last period that triggers
the sw overflow.

Signed-off-by: Robert Richter <robert.richter@amd.com>
---
arch/x86/kernel/cpu/perf_event_amd_ibs.c | 15 +++++++++++++--
1 files changed, 13 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/cpu/perf_event_amd_ibs.c b/arch/x86/kernel/cpu/perf_event_amd_ibs.c
index 1f53f16..f0271dd 100644
--- a/arch/x86/kernel/cpu/perf_event_amd_ibs.c
+++ b/arch/x86/kernel/cpu/perf_event_amd_ibs.c
@@ -85,8 +85,19 @@ perf_event_set_period(struct hw_perf_event *hwc, u64 min, u64 max, u64 *hw_perio
overflow = 1;
}

- if (left > max)
- left = max;
+ /*
+ * If the hw period that triggers the sw overflow is too short
+ * we might hit the irq handler. This biases the results.
+ * Thus we shorten the next-to-last period and set the last
+ * period to the max period.
+ */
+ if (left > max) {
+ left -= max;
+ if (left > max)
+ left = max;
+ else if (left < min)
+ left = min;
+ }

*hw_period = (u64)left;

--
1.7.8.4



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