linux-kernel - [PATCH 5.4 41/83] sched/core: Fix reset-on-fork from RT with uclamp

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Date:   Fri,  1 May 2020 15:23:20 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Chitti Babu Theegala <ctheegal@...eaurora.org>,
        Quentin Perret <qperret@...gle.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: [PATCH 5.4 41/83] sched/core: Fix reset-on-fork from RT with uclamp

From: Quentin Perret <qperret@...gle.com>

commit eaf5a92ebde5bca3bb2565616115bd6d579486cd upstream.

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_task() in
uclamp_fork() as this doesn't make sense if the reset-on-fork flag is
set.

Fixes: 1a00d999971c ("sched/uclamp: Set default clamps for RT tasks")
Reported-by: Chitti Babu Theegala <ctheegal@...eaurora.org>
Signed-off-by: Quentin Perret <qperret@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Patrick Bellasi <patrick.bellasi@...bug.net>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
Link: https://lkml.kernel.org/r/20200416085956.217587-1-qperret@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/sched/core.c |    9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1233,13 +1233,8 @@ static void uclamp_fork(struct task_stru
 		return;
 
 	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);
+		uclamp_se_set(&p->uclamp_req[clamp_id],
+			      uclamp_none(clamp_id), false);
 	}
 }