sched: Optimize task_sched_runtime()

Large multi-threaded apps like to hit this using do_sys_times() and
then queue up on the rq->lock.

Avoid when possible.

Larry reported ~20% performance increase his test case.

Reported-by: Larry Woodman <lwoodman@redhat.com>
Suggested-by: Paul Turner <pjt@google.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20131111172925.GG26898@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 1deccd78be98..c1808606ee5f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2253,6 +2253,20 @@ unsigned long long task_sched_runtime(struct task_struct *p)
         struct rq *rq;
         u64 ns = 0;
 
+#if defined(CONFIG_64BIT) && defined(CONFIG_SMP)
+        /*
+         * 64-bit doesn't need locks to atomically read a 64bit value.
+         * So we have a optimization chance when the task's delta_exec is 0.
+         * Reading ->on_cpu is racy, but this is ok.
+         *
+         * If we race with it leaving cpu, we'll take a lock. So we're correct.
+         * If we race with it entering cpu, unaccounted time is 0. This is
+         * indistinguishable from the read occurring a few cycles earlier.
+         */
+        if (!p->on_cpu)
+                return p->se.sum_exec_runtime;
+#endif
+
         rq = task_rq_lock(p, &flags);
         ns = p->se.sum_exec_runtime + do_task_delta_exec(p, rq);
         task_rq_unlock(rq, p, &flags);