linux-kernel - [PATCH] sched/fair: Fix the scene where group's imbalance is set incorrectly

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Date:   Sun, 10 Apr 2022 12:58:36 +0800
From:   zgpeng <zgpeng.linux@...il.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, linux-kernel@...r.kernel.org
Subject: [PATCH] sched/fair: Fix the scene where group's imbalance is set incorrectly

In the load_balance function, if the balance fails due to
affinity,then parent group's imbalance will be set to 1.
However, there will be a scene where balance is achieved,
but the imbalance flag is still set to 1, which needs to
be fixed.

The specific trigger scenarios are as follows. In the
load_balance function, the first loop picks out the busiest
cpu. During the process of pulling the process from the
busiest cpu, it is found that all tasks cannot be run on the
DST cpu. At this time, both LBF_SOME_PINNED andLBF_ALL_PINNED
of env.flags are set. Because balance fails and LBF_SOME_PINNED
is set, the parent group's mbalance flag will be set to 1. At
the same time, because LBF_ALL_PINNED is set, it will re-enter
the second cycle to select another busiest cpu to pull the process.
Because the busiest CPU has changed, the process can be pulled to
DST cpu, so it is possible to reach a balance state.

But at this time, the parent group's imbalance flag is not set
correctly again. As a result, the load balance is successfully
achieved, but the parent group's imbalance flag is incorrectly
set to 1. In this case, the upper-layer load balance will
erroneously select this domain as the busiest group, thereby
breaking the balance.

Signed-off-by: zgpeng <zgpeng@...cent.com>
---
 kernel/sched/fair.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d4bd299..e137917 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10019,13 +10019,13 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 		}

 		/*
-		 * We failed to reach balance because of affinity.
+		 * According to balance status, set group_imbalance correctly.
 		 */
 		if (sd_parent) {
 			int *group_imbalance = &sd_parent->groups->sgc->imbalance;

-			if ((env.flags & LBF_SOME_PINNED) && env.imbalance > 0)
-				*group_imbalance = 1;
+			if (env.flags & LBF_SOME_PINNED)
+				*group_imbalance = env.imbalance > 0 ? 1 : 0;
 		}

 		/* All tasks on this runqueue were pinned by CPU affinity */
-- 
2.9.5