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    Subject[tip:perf/core] perf: Optimize perf_event_mmap_ctx()
    Commit-ID:  f6d9dd237da400effb265f3554c64413f8a3e7b4
    Gitweb: http://git.kernel.org/tip/f6d9dd237da400effb265f3554c64413f8a3e7b4
    Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
    AuthorDate: Fri, 20 Nov 2009 22:19:48 +0100
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Sat, 21 Nov 2009 14:11:37 +0100

    perf: Optimize perf_event_mmap_ctx()

    Remove a rcu_read_{,un}lock() pair and a few conditionals.

    We can remove the rcu_read_lock() by increasing the scope of one
    in the calling function.

    We can do away with the system_state check if the machine still
    boots after this patch (seems to be the case).

    We can do away with the list_empty() check because the bare
    list_for_each_entry_rcu() reduces to that now that we've removed
    everything else.

    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Paul Mackerras <paulus@samba.org>
    LKML-Reference: <20091120212508.606459548@chello.nl>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    kernel/perf_event.c | 7 +------
    1 files changed, 1 insertions(+), 6 deletions(-)

    diff --git a/kernel/perf_event.c b/kernel/perf_event.c
    index 4deefaa..68fbf4f 100644
    --- a/kernel/perf_event.c
    +++ b/kernel/perf_event.c
    @@ -3493,15 +3493,10 @@ static void perf_event_mmap_ctx(struct perf_event_context *ctx,
    {
    struct perf_event *event;

    - if (system_state != SYSTEM_RUNNING || list_empty(&ctx->event_list))
    - return;
    -
    - rcu_read_lock();
    list_for_each_entry_rcu(event, &ctx->event_list, event_entry) {
    if (perf_event_mmap_match(event, mmap_event))
    perf_event_mmap_output(event, mmap_event);
    }
    - rcu_read_unlock();
    }

    static void perf_event_mmap_event(struct perf_mmap_event *mmap_event)
    @@ -3557,11 +3552,11 @@ got_name:

    mmap_event->event_id.header.size = sizeof(mmap_event->event_id) + size;

    + rcu_read_lock();
    cpuctx = &get_cpu_var(perf_cpu_context);
    perf_event_mmap_ctx(&cpuctx->ctx, mmap_event);
    put_cpu_var(perf_cpu_context);

    - rcu_read_lock();
    /*
    * doesn't really matter which of the child contexts the
    * events ends up in.

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