Re: [PATCH 3/4] perf/core: introduce PMU_EV_CAP_READ_ACTIVE_PKG

[Nilay Vaish <nilayvaish@gmail.com>]

On 9 August 2016 at 17:28, David Carrillo-Cisneros <davidcc@google.com> wrote:
> Introduce the flag PMU_EV_CAP_READ_ACTIVE_PKG, useful for uncore events,
> that allows a PMU to signal the generic perf code that an event is readable
> in the current CPU if the event is active in a CPU in the same package as
> the current CPU.
>
> This is an optimization that avoids a unnecessary IPI for the common case
> where uncore events are run and read in the same package but in
> different CPUs.
>
> As an example, the IPI removal speeds up perf_read in my Haswell system
> as follows:
>   - For event UNC_C_LLC_LOOKUP: From 260 us to 31 us.
>   - For event RAPL's power/energy-cores/: From to 255 us to 27 us.
>
> For the optimization to work, all events in the group must have it
> (similarly to PERF_EV_CAP_SOFTWARE).
>
> Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
> ---
>  include/linux/perf_event.h |  3 +++
>  kernel/events/core.c       | 26 ++++++++++++++++++++++++--
>  2 files changed, 27 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index fa5617f..c8bb1b3 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -501,8 +501,11 @@ typedef void (*perf_overflow_handler_t)(struct perf_event *,
>   * Event capabilities. For event_caps and groups caps.
>   *
>   * PERF_EV_CAP_SOFTWARE: Is a software event.
> + * PERF_EV_CAP_READ_ACTIVE_PKG: A CPU event (or cgroup event) that can be read
> + * from any CPU in the package where it is active.
>   */
>  #define PERF_EV_CAP_SOFTWARE           BIT(0)
> +#define PERF_EV_CAP_READ_ACTIVE_PKG    BIT(1)
>
>  #define SWEVENT_HLIST_BITS             8
>  #define SWEVENT_HLIST_SIZE             (1 << SWEVENT_HLIST_BITS)
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 34049cc..38ec68d 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3333,6 +3333,22 @@ struct perf_read_data {
>         int ret;
>  };
>
> +static int find_cpu_to_read(struct perf_event *event, int local_cpu)
> +{
> +       int event_cpu = event->oncpu;
> +       u16 local_pkg, event_pkg;
> +
> +       if (event->group_caps & PERF_EV_CAP_READ_ACTIVE_PKG) {
> +               event_pkg =  topology_physical_package_id(event_cpu);
> +               local_pkg =  topology_physical_package_id(local_cpu);
> +
> +               if (event_pkg == local_pkg)
> +                       return local_cpu;
> +       }
> +
> +       return event_cpu;
> +}
> +
>  /*
>   * Cross CPU call to read the hardware event
>   */
> @@ -3454,7 +3470,7 @@ u64 perf_event_read_local(struct perf_event *event)
>
>  static int perf_event_read(struct perf_event *event, bool group)
>  {
> -       int ret = 0;
> +       int ret = 0, cpu_to_read, local_cpu;
>
>         /*
>          * If event is enabled and currently active on a CPU, update the
> @@ -3466,8 +3482,14 @@ static int perf_event_read(struct perf_event *event, bool group)
>                         .group = group,
>                         .ret = 0,
>                 };
> -               ret = smp_call_function_single(event->oncpu,
> +
> +               local_cpu = get_cpu();
> +               cpu_to_read = find_cpu_to_read(event, local_cpu);
> +               put_cpu();
> +
> +               ret = smp_call_function_single(cpu_to_read,
>                                                __perf_event_read, &data, 1);
> +
>                 ret = ret ? : data.ret;
>         } else if (event->state == PERF_EVENT_STATE_INACTIVE) {
>                 struct perf_event_context *ctx = event->ctx;
> --
> 2.8.0.rc3.226.g39d4020
>

David, thanks for making the changes.  This patch looks good to me.

--
Nilay