Re: [PATCH v2 1/5] perf: Allow periodic events to alternate between two sample periods

[Rob Herring]

On Mon, Jan 6, 2025 at 6:12 AM <mark.barnett@xxxxxxx> wrote:
>
> From: Ben Gainey <ben.gainey@xxxxxxx>
>
> This change modifies perf_event_attr to add a second, alternative
> sample period field, and modifies the core perf overflow handling
> such that when specified an event will alternate between two sample
> periods.
>
> Currently, perf does not provide a  mechanism for decoupling the period
> over which counters are counted from the period between samples. This is
> problematic for building a tool to measure per-function metrics derived
> from a sampled counter group. Ideally such a tool wants a very small
> sample window in order to correctly attribute the metrics to a given
> function, but prefers a larger sample period that provides representative
> coverage without excessive probe effect, triggering throttling, or
> generating excessive amounts of data.
>
> By alternating between a long and short sample_period and subsequently
> discarding the long samples, tools may decouple the period between
> samples that the tool cares about from the window of time over which
> interesting counts are collected.
>
> It is expected that typically tools would use this feature with the
> cycles or instructions events as an approximation for time, but no
> restrictions are applied to which events this can be applied to.
>
> Signed-off-by: Ben Gainey <ben.gainey@xxxxxxx>
> Signed-off-by: Mark Barnett <mark.barnett@xxxxxxx>
> ---
>  include/linux/perf_event.h      |  5 +++++
>  include/uapi/linux/perf_event.h |  3 +++
>  kernel/events/core.c            | 37 ++++++++++++++++++++++++++++++++-
>  3 files changed, 44 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index cb99ec8c9e96..cbb332f4e19c 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -276,6 +276,11 @@ struct hw_perf_event {
>          */
>         u64                             freq_time_stamp;
>         u64                             freq_count_stamp;
> +
> +       /*
> +        * Indicates that the alternative sample period is used
> +        */
> +       bool                            using_alt_sample_period;

8 bytes more for a single bit of data. I think we can avoid it. More below.

>  #endif
>  };
>
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 0524d541d4e3..499a8673df8e 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -379,6 +379,7 @@ enum perf_event_read_format {
>  #define PERF_ATTR_SIZE_VER6    120     /* add: aux_sample_size */
>  #define PERF_ATTR_SIZE_VER7    128     /* add: sig_data */
>  #define PERF_ATTR_SIZE_VER8    136     /* add: config3 */
> +#define PERF_ATTR_SIZE_VER9    144     /* add: alt_sample_period */
>
>  /*
>   * Hardware event_id to monitor via a performance monitoring event:
> @@ -531,6 +532,8 @@ struct perf_event_attr {
>         __u64   sig_data;
>
>         __u64   config3; /* extension of config2 */
> +
> +       __u64   alt_sample_period;
>  };
>
>  /*
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 065f9188b44a..7e339d12363a 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4178,6 +4178,8 @@ static void perf_adjust_period(struct perf_event *event, u64 nsec, u64 count, bo
>         s64 period, sample_period;
>         s64 delta;
>
> +       WARN_ON_ONCE(hwc->using_alt_sample_period);
> +
>         period = perf_calculate_period(event, nsec, count);
>
>         delta = (s64)(period - hwc->sample_period);
> @@ -9850,6 +9852,7 @@ static int __perf_event_overflow(struct perf_event *event,
>                                  int throttle, struct perf_sample_data *data,
>                                  struct pt_regs *regs)
>  {
> +       struct hw_perf_event *hwc = &event->hw;
>         int events = atomic_read(&event->event_limit);
>         int ret = 0;
>
> @@ -9869,6 +9872,18 @@ static int __perf_event_overflow(struct perf_event *event,
>             !bpf_overflow_handler(event, data, regs))
>                 goto out;
>
> +       /*
> +        * Swap the sample period to the alternative period
> +        */
> +       if (event->attr.alt_sample_period) {
> +               bool using_alt = hwc->using_alt_sample_period;
> +               u64 sample_period = (using_alt ? event->attr.sample_period
> +                                              : event->attr.alt_sample_period);
> +
> +               hwc->sample_period = sample_period;
> +               hwc->using_alt_sample_period = !using_alt;
> +       }

Wouldn't something like this avoid the need for using_alt_sample_period:

if (event->attr.alt_sample_period) {
  if (hwc->sample_period == event->attr.sample_period)
    hwc->sample_period = event->attr.alt_sample_period;
  else
    hwc->sample_period = event->attr.sample_period;
}

Rob