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

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