perf: Optimize perf_event_comm_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: <20091120212508.527608793@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 2afb305944af4b6ac91442a64156b366d6aec59f..4deefaace90e962315a45b875d7700ca08ef6b8f 100644 (file)
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -3374,15 +3374,10 @@ static void perf_event_comm_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_comm_match(event))
                        perf_event_comm_output(event, comm_event);
        }
-       rcu_read_unlock();
 }
 
 static void perf_event_comm_event(struct perf_comm_event *comm_event)
@@ -3401,11 +3396,11 @@ static void perf_event_comm_event(struct perf_comm_event *comm_event)
 
        comm_event->event_id.header.size = sizeof(comm_event->event_id) + size;
 
+       rcu_read_lock();
        cpuctx = &get_cpu_var(perf_cpu_context);
        perf_event_comm_ctx(&cpuctx->ctx, comm_event);
        put_cpu_var(perf_cpu_context);
 
-       rcu_read_lock();
        /*
         * doesn't really matter which of the child contexts the
         * events ends up in.