sched/fair: Prevent cfs_rq from being unthrottled with zero runtime_remaining

When a cfs_rq is to be throttled, its limbo list should be empty and
that's why there is a warn in tg_throttle_down() for non empty
cfs_rq->throttled_limbo_list.

When running a test with the following hierarchy:

          root
        /      \
        A*     ...
     /  |  \   ...
        B
       /  \
      C*

where both A and C have quota settings, that warn on non empty limbo list
is triggered for a cfs_rq of C, let's call it cfs_rq_c(and ignore the cpu
part of the cfs_rq for the sake of simpler representation).

Debug showed it happened like this:
Task group C is created and quota is set, so in tg_set_cfs_bandwidth(),
cfs_rq_c is initialized with runtime_enabled set, runtime_remaining
equals to 0 and *unthrottled*. Before any tasks are enqueued to cfs_rq_c,
*multiple* throttled tasks can migrate to cfs_rq_c (e.g., due to task
group changes). When enqueue_task_fair(cfs_rq_c, throttled_task) is
called and cfs_rq_c is in a throttled hierarchy (e.g., A is throttled),
these throttled tasks are directly placed into cfs_rq_c's limbo list by
enqueue_throttled_task().

Later, when A is unthrottled, tg_unthrottle_up(cfs_rq_c) enqueues these
tasks. The first enqueue triggers check_enqueue_throttle(), and with zero
runtime_remaining, cfs_rq_c can be throttled in throttle_cfs_rq() if it
can't get more runtime and enters tg_throttle_down(), where the warning
is hit due to remaining tasks in the limbo list.

I think it's a chaos to trigger throttle on unthrottle path, the status
of a being unthrottled cfs_rq can be in a mixed state in the end, so fix
this by granting 1ns to cfs_rq in tg_set_cfs_bandwidth(). This ensures
cfs_rq_c has a positive runtime_remaining when initialized as unthrottled
and cannot enter tg_unthrottle_up() with zero runtime_remaining.

Also, update outdated comments in tg_throttle_down() since
unthrottle_cfs_rq() is no longer called with zero runtime_remaining.
While at it, remove a redundant assignment to se in tg_throttle_down().

Fixes: e1fad12 ("sched/fair: Switch to task based throttle model")
Reviewed-By: Benjamin Segall <bsegall@google.com>
Suggested-by: Benjamin Segall <bsegall@google.com>
Signed-off-by: Aaron Lu <ziqianlu@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: Hao Jia <jiahao1@lixiang.com>
Link: https://patch.msgid.link/20251030032755.560-1-ziqianlu@bytedance.com

Author: aaronlu

kernel/sched/core.c

@@ -9606,7 +9606,7 @@ static int tg_set_cfs_bandwidth(struct task_group *tg,
 
 		guard(rq_lock_irq)(rq);
 		cfs_rq->runtime_enabled = runtime_enabled;
-		cfs_rq->runtime_remaining = 0;
+		cfs_rq->runtime_remaining = 1;
 
 		if (cfs_rq->throttled)
 			unthrottle_cfs_rq(cfs_rq);

kernel/sched/fair.c

@@ -6024,20 +6024,17 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
 	struct sched_entity *se = cfs_rq->tg->se[cpu_of(rq)];
 
 	/*
-	 * It's possible we are called with !runtime_remaining due to things
-	 * like user changed quota setting(see tg_set_cfs_bandwidth()) or async
-	 * unthrottled us with a positive runtime_remaining but other still
-	 * running entities consumed those runtime before we reached here.
+	 * It's possible we are called with runtime_remaining < 0 due to things
+	 * like async unthrottled us with a positive runtime_remaining but other
+	 * still running entities consumed those runtime before we reached here.
 	 *
-	 * Anyway, we can't unthrottle this cfs_rq without any runtime remaining
-	 * because any enqueue in tg_unthrottle_up() will immediately trigger a
-	 * throttle, which is not supposed to happen on unthrottle path.
+	 * We can't unthrottle this cfs_rq without any runtime remaining because
+	 * any enqueue in tg_unthrottle_up() will immediately trigger a throttle,
+	 * which is not supposed to happen on unthrottle path.
 	 */
 	if (cfs_rq->runtime_enabled && cfs_rq->runtime_remaining <= 0)
 		return;
 
-	se = cfs_rq->tg->se[cpu_of(rq)];
-
 	cfs_rq->throttled = 0;
 
 	update_rq_clock(rq);