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    Subject[tip:perf/core] perf/core: Define PERF_PMU_TXN_READ interface
    Commit-ID:  4a00c16e552ea5e71756cd29cd2df7557ec9cac4
    Gitweb: http://git.kernel.org/tip/4a00c16e552ea5e71756cd29cd2df7557ec9cac4
    Author: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
    AuthorDate: Thu, 3 Sep 2015 20:07:51 -0700
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Sun, 13 Sep 2015 11:27:28 +0200

    perf/core: Define PERF_PMU_TXN_READ interface

    Define a new PERF_PMU_TXN_READ interface to read a group of counters
    at once.

    pmu->start_txn() // Initialize before first event

    for each event in group
    pmu->read(event); // Queue each event to be read

    rc = pmu->commit_txn() // Read/update all queued counters

    Note that we use this interface with all PMUs. PMUs that implement this
    interface use the ->read() operation to _queue_ the counters to be read
    and use ->commit_txn() to actually read all the queued counters at once.

    PMUs that don't implement PERF_PMU_TXN_READ ignore ->start_txn() and
    ->commit_txn() and continue to read counters one at a time.

    Thanks to input from Peter Zijlstra.

    Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
    Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
    Cc: Jiri Olsa <jolsa@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Michael Ellerman <mpe@ellerman.id.au>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Stephane Eranian <eranian@google.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Vince Weaver <vincent.weaver@maine.edu>
    Link: http://lkml.kernel.org/r/1441336073-22750-9-git-send-email-sukadev@linux.vnet.ibm.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    include/linux/perf_event.h | 1 +
    kernel/events/core.c | 24 +++++++++++++++++++-----
    2 files changed, 20 insertions(+), 5 deletions(-)

    diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
    index ea3b5dd..b83cea9 100644
    --- a/include/linux/perf_event.h
    +++ b/include/linux/perf_event.h
    @@ -202,6 +202,7 @@ struct perf_event;
    #define PERF_EVENT_TXN 0x1

    #define PERF_PMU_TXN_ADD 0x1 /* txn to add/schedule event on PMU */
    +#define PERF_PMU_TXN_READ 0x2 /* txn to read event group from PMU */

    /**
    * pmu::capabilities flags
    diff --git a/kernel/events/core.c b/kernel/events/core.c
    index ade04df..55b0f7c 100644
    --- a/kernel/events/core.c
    +++ b/kernel/events/core.c
    @@ -3199,6 +3199,7 @@ static void __perf_event_read(void *info)
    struct perf_event *sub, *event = data->event;
    struct perf_event_context *ctx = event->ctx;
    struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
    + struct pmu *pmu = event->pmu;

    /*
    * If this is a task context, we need to check whether it is
    @@ -3217,18 +3218,31 @@ static void __perf_event_read(void *info)
    }

    update_event_times(event);
    - if (event->state == PERF_EVENT_STATE_ACTIVE)
    - event->pmu->read(event);
    + if (event->state != PERF_EVENT_STATE_ACTIVE)
    + goto unlock;

    - if (!data->group)
    + if (!data->group) {
    + pmu->read(event);
    + data->ret = 0;
    goto unlock;
    + }
    +
    + pmu->start_txn(pmu, PERF_PMU_TXN_READ);
    +
    + pmu->read(event);

    list_for_each_entry(sub, &event->sibling_list, group_entry) {
    update_event_times(sub);
    - if (sub->state == PERF_EVENT_STATE_ACTIVE)
    + if (sub->state == PERF_EVENT_STATE_ACTIVE) {
    + /*
    + * Use sibling's PMU rather than @event's since
    + * sibling could be on different (eg: software) PMU.
    + */
    sub->pmu->read(sub);
    + }
    }
    - data->ret = 0;
    +
    + data->ret = pmu->commit_txn(pmu);

    unlock:
    raw_spin_unlock(&ctx->lock);

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