[PATCH] sched/fair: update_curr changed sum_exec_runtime to 1 when sum_exec_runtime is 0 beacuse some kernel code use sum_exec_runtime==0 to test task just be forked.

[QiaoChong]

From: Chong Qiao <qiaochong@xxxxxxxxxxx>

Such as:
cpu_cgroup_attach>
 sched_move_task>
  task_change_group_fair>
   task_move_group_fair>
    detach_task_cfs_rq>
     vruntime_normalized>

	/*
	 * When !on_rq, vruntime of the task has usually NOT been normalized.
	 * But there are some cases where it has already been normalized:
	 *
	 * - A forked child which is waiting for being woken up by
	 *   wake_up_new_task().
	 * - A task which has been woken up by try_to_wake_up() and
	 *   waiting for actually being woken up by sched_ttwu_pending().
	 */
	if (!se->sum_exec_runtime ||
	    (p->state == TASK_WAKING && p->sched_remote_wakeup))
		return true;

p->se.sum_exec_runtime is 0, does not mean task not been run (A forked child which is waiting for being woken up by  wake_up_new_task()).

Task may have been scheduled multimes, but p->se.sum_exec_runtime is still 0, because delta_exec maybe 0 in update_curr.

static void update_curr(struct cfs_rq *cfs_rq)
{
...
	delta_exec = now - curr->exec_start;
	if (unlikely((s64)delta_exec <= 0))
		return;
...

	curr->sum_exec_runtime += delta_exec;
...
}

Task has been run and is stopped(on_rq == 0), vruntime not been normalized, but se->sum_exec_runtime == 0.
This cause vruntime_normalized set on_rq 1, and does not normalize vruntime.
This may cause task use old vruntime in old cgroup, which maybe very large than task's vruntime in new cgroup.
Which may cause task may not scheduled in run queue for long time after been waked up.

Now I change sum_exec_runtime to 1 when sum_exec_runtime == 0 in update_curr to make sun_exec_runtime not 0.

Signed-off-by: Chong Qiao <qiaochong@xxxxxxxxxxx>
---
 kernel/sched/fair.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bc9cfeaac8bd2..c4cd6249d9d2c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -841,8 +841,15 @@ static void update_curr(struct cfs_rq *cfs_rq)
 		return;
 
 	delta_exec = now - curr->exec_start;
-	if (unlikely((s64)delta_exec <= 0))
+	if (unlikely((s64)delta_exec <= 0)) {
+		/*
+		 * se.sum_exec_runtime == 0 means task is forked and wait to be run.
+		 * So here change se.sum_exec_runtime to 1, to make it not 0.
+		 */
+		if (!curr->sum_exec_runtime)
+			curr->sum_exec_runtime++;
 		return;
+	}
 
 	curr->exec_start = now;
 
-- 
2.17.1