From 2630cde26711dab0d0b56a8be1616475be646d13 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Michal=20Koutn=C3=BD?= Date: Fri, 17 Sep 2021 17:30:37 +0200 Subject: [PATCH] sched/fair: Add ancestors of unthrottled undecayed cfs_rq MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Vincent Guittot Reviewed-by: Odin Ugedal 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 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;