linux-kernel - [RESEND PATCH v3 2/2] sched/deadline: Initialize cp->elements[].cpu to an invalid value

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Date:   Tue, 19 Dec 2017 10:19:24 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     peterz@...radead.org, mingo@...nel.org, rostedt@...dmis.org
Cc:     tglx@...utronix.de, raistlin@...ux.it,
        linux-kernel@...r.kernel.org, juri.lelli@...il.com,
        bristot@...hat.com, kernel-team@....com
Subject: [RESEND PATCH v3 2/2] sched/deadline: Initialize cp->elements[].cpu to an invalid value

Currently, migrating tasks to cpu0 unconditionally happens when the
heap is empty, since cp->elements[].cpu was initialized to 0(=cpu0).
We have to distinguish between the empty case and cpu0 to avoid the
unnecessary migrations. Therefore, it has to return an invalid value
e.i. -1 in that case.

Signed-off-by: Byungchul Park <byungchul.park@....com>
Acked-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
Acked-by: Daniel Bristot de Oliveira <bristot@...hat.com>
---
 kernel/sched/cpudeadline.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
index 9f02035..bcf903f 100644
--- a/kernel/sched/cpudeadline.c
+++ b/kernel/sched/cpudeadline.c
@@ -138,6 +138,12 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
 		int best_cpu = cpudl_maximum_cpu(cp);
 		WARN_ON(best_cpu != -1 && !cpu_present(best_cpu));
 
+		/*
+		 * The heap tree is empty for now, just return.
+		 */
+		if (best_cpu == -1)
+			return 0;
+
 		if (cpumask_test_cpu(best_cpu, &p->cpus_allowed) &&
 		    dl_time_before(dl_se->deadline, cpudl_maximum_dl(cp))) {
 			if (later_mask)
@@ -265,8 +271,10 @@ int cpudl_init(struct cpudl *cp)
 		return -ENOMEM;
 	}
 
-	for_each_possible_cpu(i)
+	for_each_possible_cpu(i) {
+		cp->elements[i].cpu = -1;
 		cp->elements[i].idx = IDX_INVALID;
+	}
 
 	return 0;
 }
-- 
1.9.1