[Devel] [PATCH vz9] sched: Do not set LBF_NEED_BREAK flag if scanned all the tasks

[Konstantin Khorenko]

After ms commit b0defa7ae03e ("sched/fair: Make sure to try to detach at
least one movable task") detach_tasks() does not stop on the condition
(env->loop > env->loop_max) in case no movable task found.

Instead of that (if there are no movable tasks in the rq) exits always
happen on the loop_break check - thus with LBF_NEED_BREAK flag set.

It's not a problem for mainstream because load_balance() proceeds with
balancing in case LBF_NEED_BREAK is set only in case (env.loop <
busiest->nr_running), but in Virtuozzo kernel with CFS_CPULIMIT feature
right before that we try to move a whole task group (object of the
CFS_CPULIMIT feature) and resets env.loop to zero,
so we get a livelock here.

Resetting env.loop makes sense in case we have successfully moved some
tasks (in the scope of the task group), but if we failed to move any
task, no progress is expected during further balancing attempts.

Ways to fix it:
 1. In load_balance() restore old env.loop in case no tasks were moved
    by move_task_groups()
 2. Add a check in detach_tasks() to exit without LBF_NEED_BREAK flag in
    case we have scanned all tasks and have not found movable tasks.

Current patch implements the second way.

Caused by ms commit: b0defa7ae03e ("sched/fair: Make sure to try to
detach at least one movable task")

Fixes ms commit: bca010328248 ("sched: Port CONFIG_CFS_CPULIMIT feature")

Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>

Feature: sched: ability to limit number of CPUs available to a CT
---
 kernel/sched/fair.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9367d16a8d85..e068bb90f197 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8771,6 +8771,12 @@ static int detach_tasks(struct lb_env *env)
 		if (env->loop > env->loop_max &&
 		    !(env->flags & LBF_ALL_PINNED))
 			break;
+		/*
+		 * Quit if we have scanned all tasks even in case we haven't
+		 * found any movable task.
+		 */
+		if (env->loop > env->src_rq->nr_running)
+			break;
 
 		/* take a breather every nr_migrate tasks */
 		if (env->loop > env->loop_break) {
-- 
2.39.3