Commit 7da9960b authored Apr 24, 2025 by Kan Liang Committed by Peter Zijlstra Apr 25, 2025

perf/x86/intel/ds: Fix counter backwards of non-precise events counters-snapshotting



The counter backwards may be observed in the PMI handler when
counters-snapshotting some non-precise events in the freq mode.

For the non-precise events, it's possible the counters-snapshotting
records a positive value for an overflowed PEBS event. Then the HW
auto-reload mechanism reset the counter to 0 immediately. Because the
pebs_event_reset is cleared in the freq mode, which doesn't set the
PERF_X86_EVENT_AUTO_RELOAD.
In the PMI handler, 0 will be read rather than the positive value
recorded in the counters-snapshotting record.

The counters-snapshotting case has to be specially handled. Since the
event value has been updated when processing the counters-snapshotting
record, only needs to set the new period for the counter via
x86_pmu_set_period().

Fixes: e02e9b03 ("perf/x86/intel: Support PEBS counters snapshotting")
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250424134718.311934-6-kan.liang@linux.intel.com

parent e9988ad7

arch/x86/events/intel/ds.c

+19 −2

Original line number	Diff line number	Diff line
		@@ -2379,9 +2379,26 @@ __intel_pmu_pebs_last_event(struct perf_event *event,
		*/
		intel_pmu_save_and_restart_reload(event, count);
		}
		} else
		} else {
		/*
		* For a non-precise event, it's possible the
		* counters-snapshotting records a positive value for the
		* overflowed event. Then the HW auto-reload mechanism
		* reset the counter to 0 immediately, because the
		* pebs_event_reset is cleared if the PERF_X86_EVENT_AUTO_RELOAD
		* is not set. The counter backwards may be observed in a
		* PMI handler.
		*
		* Since the event value has been updated when processing the
		* counters-snapshotting record, only needs to set the new
		* period for the counter.
		*/
		if (is_pebs_counter_event_group(event))
		static_call(x86_pmu_set_period)(event);
		else
		intel_pmu_save_and_restart(event);
		}
		}

		static __always_inline void
		__intel_pmu_pebs_events(struct perf_event *event,