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

From: Liang, Kan
Date: Mon Mar 24 2025 - 13:26:34 EST


Hi Peter,

The patch is to fix a counters-snapshotting backwards issue with
non-precise events in freq mode. Could you please pick it up as well?

Thanks,
Kan

On 2025-02-04 4:05 p.m., kan.liang@xxxxxxxxxxxxxxx wrote:
> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
>
> 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().
>
> Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> ---
> arch/x86/events/intel/ds.c | 21 +++++++++++++++++++--
> 1 file changed, 19 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
> index 46aaaeae0c8d..e8f808905871 100644
> --- a/arch/x86/events/intel/ds.c
> +++ b/arch/x86/events/intel/ds.c
> @@ -2377,8 +2377,25 @@ __intel_pmu_pebs_last_event(struct perf_event *event,
> */
> intel_pmu_save_and_restart_reload(event, count);
> }
> - } else
> - intel_pmu_save_and_restart(event);
> + } 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