perf/x86-ibs: Extend hw period that triggers overflow

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>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1333390758-10893-10-git-send-email-robert.richter@amd.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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 3e32908292a7..cb51a3e55870 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;