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);