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From:   Xuewen Yan <xuewen.yan@unisoc.com>
To:     <mingo@redhat.com>, <peterz@infradead.org>,
        <juri.lelli@redhat.com>, <vincent.guittot@linaro.org>,
        <dietmar.eggemann@arm.com>, <rostedt@goodmis.org>,
        <bsegall@google.com>, <mgorman@suse.de>, <bristot@redhat.com>,
        <vschneid@redhat.com>
CC:     <linux-kernel@vger.kernel.org>, <ke.wang@unisoc.com>,
        <xuewen.yan94@gmail.com>
Subject: [PATCH] sched/eevdf: Avoid NULL in pick_eevdf
Date:   Mon, 20 Nov 2023 15:38:21 +0800
Message-ID: <20231120073821.1304-1-xuewen.yan@unisoc.com>
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Precedence: bulk

Now in pick_eevdf function, add the pick_first_entity to prevent
picking null when using eevdf, however, the leftmost may be null.
As a result, it would cause oops because the se is NULL.

Fix this by compare the curr and left, if the left is null, set
the se be curr.

Fixes: 147f3efaa241 ("sched/fair: Implement an EEVDF-like scheduling policy")
Signed-off-by: Xuewen Yan <xuewen.yan@unisoc.com>
---
 kernel/sched/fair.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d7a3c63a2171..10916f6778ac 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -951,12 +951,28 @@ static struct sched_entity *__pick_eevdf(struct cfs_rq *cfs_rq)
 	return NULL;
 }
 
+/* Just simply choose the se with the smallest vruntime */
+static struct sched_entity *__pick_cfs(struct cfs_rq *cfs_rq)
+{
+	struct sched_entity *curr = cfs_rq->curr;
+	struct sched_entity *left = __pick_first_entity(cfs_rq);
+
+	/*
+	 * If curr is set we have to see if its left of the leftmost entity
+	 * still in the tree, provided there was anything in the tree at all.
+	 */
+	if (!left || (curr && entity_before(curr, left)))
+		left = curr;
+
+	return left;
+}
+
 static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
 {
 	struct sched_entity *se = __pick_eevdf(cfs_rq);
 
 	if (!se) {
-		struct sched_entity *left = __pick_first_entity(cfs_rq);
+		struct sched_entity *left = __pick_cfs(cfs_rq);
 		if (left) {
 			pr_err("EEVDF scheduling fail, picking leftmost\n");
 			return left;
-- 
2.25.1