linux-kernel - [tip: sched/urgent] sched/fair: Add ancestors of unthrottled undecayed cfs

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Message-ID: <163309024153.25758.5768319278915743585.tip-bot2@tip-bot2>
Date:   Fri, 01 Oct 2021 12:10:41 -0000
From:   tip-bot2 for Michal Koutný 
        <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     mkoutny@...e.com, "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Odin Ugedal <odin@...d.al>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: sched/urgent] sched/fair: Add ancestors of unthrottled undecayed cfs_rq

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     2630cde26711dab0d0b56a8be1616475be646d13
Gitweb:        https://git.kernel.org/tip/2630cde26711dab0d0b56a8be1616475be646d13
Author:        Michal Koutný <mkoutny@...e.com>
AuthorDate:    Fri, 17 Sep 2021 17:30:37 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Fri, 01 Oct 2021 13:57:57 +02:00

sched/fair: Add ancestors of unthrottled undecayed cfs_rq

Since commit a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to
list on unthrottle") we add cfs_rqs with no runnable tasks but not fully
decayed into the load (leaf) list. We may ignore adding some ancestors
and therefore breaking tmp_alone_branch invariant. This broke LTP test
cfs_bandwidth01 and it was partially fixed in commit fdaba61ef8a2
("sched/fair: Ensure that the CFS parent is added after unthrottling").

I noticed the named test still fails even with the fix (but with low
probability, 1 in ~1000 executions of the test). The reason is when
bailing out of unthrottle_cfs_rq early, we may miss adding ancestors of
the unthrottled cfs_rq, thus, not joining tmp_alone_branch properly.

Fix this by adding ancestors if we notice the unthrottled cfs_rq was
added to the load list.

Fixes: a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
Signed-off-by: Michal Koutný <mkoutny@...e.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
Reviewed-by: Odin Ugedal <odin@...d.al>
Link: https://lore.kernel.org/r/20210917153037.11176-1-mkoutny@suse.com
---
 kernel/sched/fair.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ff69f24..f6a05d9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4936,8 +4936,12 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
 	/* update hierarchical throttle state */
 	walk_tg_tree_from(cfs_rq->tg, tg_nop, tg_unthrottle_up, (void *)rq);

-	if (!cfs_rq->load.weight)
+	/* Nothing to run but something to decay (on_list)? Complete the branch */
+	if (!cfs_rq->load.weight) {
+		if (cfs_rq->on_list)
+			goto unthrottle_throttle;
 		return;
+	}

 	task_delta = cfs_rq->h_nr_running;
 	idle_task_delta = cfs_rq->idle_h_nr_running;