linux-kernel - [PATCH v2 1/2] sched/fair: fix propagate during synchronous attach/detach

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Message-Id: <20220526071354.6426-2-zhouchengming@bytedance.com>
Date:   Thu, 26 May 2022 15:13:53 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, odin@...d.al
Cc:     linux-kernel@...r.kernel.org, duanxiongchun@...edance.com,
        songmuchun@...edance.com,
        Chengming Zhou <zhouchengming@...edance.com>
Subject: [PATCH v2 1/2] sched/fair: fix propagate during synchronous attach/detach

When a task moves from/to a cfs_rq, we first detach/attach the load_avg
of se from/to that cfs_rq, then propagate the changes across the tg tree
to make it visible to the root, which did in update_load_avg().

But the current code will break when encountering a on_list cfs_rq,
can't propagate up to the root cfs_rq, that also mismatch with the
comment of propagate_entity_cfs_rq(), which says "Propagate the changes
of the sched_entity across the tg tree to make it visible to the root".

The second problem is that it won't update_load_avg() for throttled
cfs_rq, cause the load changes can't be propagated upwards.

    A
    |
    B  --> throttled cfs_rq
   /
  C

The prop_runnable_sum of C won't be propagated to B, so won't be
propagated to A.

Fixes: 0258bdfaff5b ("sched/fair: Fix unfairness caused by missing load decay")
Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
---
 kernel/sched/fair.c | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 523e548c8fdd..5276d05692e0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11303,14 +11303,8 @@ static void propagate_entity_cfs_rq(struct sched_entity *se)
 	for_each_sched_entity(se) {
 		cfs_rq = cfs_rq_of(se);

-		if (!cfs_rq_throttled(cfs_rq)){
-			update_load_avg(cfs_rq, se, UPDATE_TG);
-			list_add_leaf_cfs_rq(cfs_rq);
-			continue;
-		}
-
-		if (list_add_leaf_cfs_rq(cfs_rq))
-			break;
+		update_load_avg(cfs_rq, se, UPDATE_TG);
+		list_add_leaf_cfs_rq(cfs_rq);
 	}
 }
 #else
-- 
2.36.1