linux-kernel - [PATCH] sched/deadline: Skip overflow check if 0 capacity

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Message-ID: <20241108042924.520458-1-longman@redhat.com>
Date: Thu,  7 Nov 2024 23:29:24 -0500
From: Waiman Long <longman@...hat.com>
To: Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>
Cc: linux-kernel@...r.kernel.org,
	Phil Auld <pauld@...hat.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Waiman Long <longman@...hat.com>
Subject: [PATCH] sched/deadline: Skip overflow check if 0 capacity

By properly setting up a 1-cpu sched domain (partition) with no
task, it was found that offlining that particular CPU failed because
dl_bw_check_overflow() in cpuset_cpu_inactive() returned -EBUSY. This
is due to the fact that dl_bw_capacity() return 0 as the sched domain
has no active CPU causing a false positive in the overflow check.

Fix this corner case by skipping the __dl_overflow() check in
dl_bw_manage() when the returned capacity is 0.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/sched/deadline.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index be1b917dc8ce..0195f350d6d3 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -3479,7 +3479,13 @@ static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw)
 	} else {
 		unsigned long cap = dl_bw_capacity(cpu);
 
-		overflow = __dl_overflow(dl_b, cap, 0, dl_bw);
+		/*
+		 * In the unlikely case of 0 capacity (e.g. a sched domain
+		 * with no active CPUs), skip the overflow check as it will
+		 * always return a false positive.
+		 */
+		if (likely(cap))
+			overflow = __dl_overflow(dl_b, cap, 0, dl_bw);
 
 		if (req == dl_bw_req_alloc && !overflow) {
 			/*
-- 
2.47.0