linux-kernel - [PATCH v3] sched/fair: Add ancestors of unthrottled undecayed cfs

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Date:   Fri, 17 Sep 2021 17:30:37 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     linux-kernel@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Phil Auld <pauld@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Odin Ugedal <odin@...d.al>, Rik van Riel <riel@...riel.com>,
        Giovanni Gherdovich <ggherdovich@...e.cz>
Subject: [PATCH v3] 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>
---

Changes since v2 [1]:
- jump to completion for loop, don't duplicate it
- singled out of the series (to handle a fix separately from the rest)

[1] https://lore.kernel.org/r/20210819175034.4577-2-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 ff69f245b939..f6a05d9b5443 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;
-- 
2.32.0