From: KOSAKI Motohiro <kosaki.motoh...@jp.fujitsu.com>
rq lock in task_sched_runtime() is necessary for two reasons. 1)
accessing se.sum_exec_runtime is not atomic on 32bit and 2)
do_task_delta_exec() require it.
So, 64bit can avoid holding rq lock when add_delta is false and
delta_exec is 0.
Cc: Olivier Langlois <oliv...@trillion01.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Frederic Weisbecker <fweis...@gmail.com>
Cc: Ingo Molnar <mi...@kernel.org>
Suggested-by: Paul Turner <p...@google.com>
Acked-by: Peter Zijlstra <pet...@infradead.org>
Signed-off-by: KOSAKI Motohiro <kosaki.motoh...@jp.fujitsu.com>
---
kernel/sched/core.c | 15 +++++++++++++++
1 files changed, 15 insertions(+), 0 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b817e6d..75872c3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2657,6 +2657,21 @@ unsigned long long task_sched_runtime(struct task_struct
*p, bool add_delta)
struct rq *rq;
u64 ns = 0;
+#ifdef CONFIG_64BIT
+ /*
+ * 64-bit doesn't need locks to atomically read a 64bit value. So we
+ * have two optimization chances, 1) when caller doesn't need
+ * delta_exec and 2) when the task's delta_exec is 0. The former is
+ * obvious. The latter is complicated. 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 (!add_delta || !p->on_cpu)
+ return p->se.sum_exec_runtime;
+#endif
+
rq = task_rq_lock(p, &flags);
ns = p->se.sum_exec_runtime;
if (add_delta)
--
1.7.1
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