linux-kernel - [PATCH] sched/core: Fix reset-on-fork from RT with uclamp

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Message-Id: <20200414161320.251897-1-qperret@google.com>
Date:   Tue, 14 Apr 2020 17:13:20 +0100
From:   Quentin Perret <qperret@...gle.com>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org
Cc:     dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, ctheegal@...eaurora.org, dianders@...omium.org,
        patrick.bellasi@...bug.net, valentin.schneider@....com,
        qais.yousef@....com, linux-kernel@...r.kernel.org,
        qperret@...gle.com, kernel-team@...roid.com
Subject: [PATCH] sched/core: Fix reset-on-fork from RT with uclamp

uclamp_fork() resets the uclamp values to their default when the
reset-on-fork flag is set. It also checks whether the task has a RT
policy, and sets its uclamp.min to 1024 accordingly. However, during
reset-on-fork, the task's policy is lowered to SCHED_NORMAL right after,
hence leading to an erroneous uclamp.min setting for the new task if it
was forked from RT.

Fix this by removing the unnecessary check on rt_policy() in
uclamp_fork() as this doesn't make sense if the reset-on-fork flag is
set.

Reported-by: Chitti Babu Theegala <ctheegal@...eaurora.org>
Signed-off-by: Quentin Perret <qperret@...gle.com>
---
 kernel/sched/core.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3a61a3b8eaa9..9ea3e484eea2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1234,10 +1234,6 @@ static void uclamp_fork(struct task_struct *p)
 	for_each_clamp_id(clamp_id) {
 		unsigned int clamp_value = uclamp_none(clamp_id);
 
-		/* By default, RT tasks always get 100% boost */
-		if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
-			clamp_value = uclamp_none(UCLAMP_MAX);
-
 		uclamp_se_set(&p->uclamp_req[clamp_id], clamp_value, false);
 	}
 }
-- 
2.26.0.110.g2183baf09c-goog