linux-kernel - [PATCH] sched/fair: Fix update_min_vruntime() to get proper min

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Date:   Tue,  6 Sep 2016 14:05:17 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     peterz@...radead.org, mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH] sched/fair: Fix update_min_vruntime() to get proper min_vruntime

I am sorry for making you confused. The patch I posted one year ago,
Commit 97a7142 'sched/fair: Make update_min_vruntime() more readable',
has a bug, which you picked up.

Please add this patch to fix it, or combine these two patches into
one, or revert commit 97a7142. Whatever you want. Sorry.

Thank you,
Byungchul

-----8<-----
>From 295895be8befbab040d6054bb8186c03daabcedd Mon Sep 17 00:00:00 2001
From: Byungchul Park <byungchul.park@....com>
Date: Tue, 6 Sep 2016 12:22:26 +0900
Subject: [PATCH] sched/fair: Fix update_min_vruntime() to get proper
 min_vruntime

Commit 97a7142 'sched/fair: Make update_min_vruntime() more readable'
introduces a bug that cfs_rq gets a wrong min_vruntime if
!cfs_rq->rb_leftmost && cfs_rq->curr. This fixes it and makes it more
readable and simple.

Signed-off-by: Byungchul Park <byungchul.park@....com>
---
 kernel/sched/fair.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a6820b3..0a5f666b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -462,21 +462,20 @@ static inline int entity_before(struct sched_entity *a,
 
 static void update_min_vruntime(struct cfs_rq *cfs_rq)
 {
-	u64 vruntime = cfs_rq->min_vruntime;
-
-	if (cfs_rq->rb_leftmost) {
-		struct sched_entity *se = rb_entry(cfs_rq->rb_leftmost,
-						   struct sched_entity,
-						   run_node);
+	u64 vruntime = U64_MAX;
 
-		vruntime = se->vruntime;
-	}
+	if (cfs_rq->rb_leftmost)
+		vruntime = rb_entry(cfs_rq->rb_leftmost,
+				    struct sched_entity,
+				    run_node)->vruntime;
 
 	if (cfs_rq->curr)
 		vruntime = min_vruntime(vruntime, cfs_rq->curr->vruntime);
 
 	/* ensure we never gain time by being placed backwards. */
-	cfs_rq->min_vruntime = max_vruntime(cfs_rq->min_vruntime, vruntime);
+	if (vruntime != U64_MAX)
+		cfs_rq->min_vruntime = max_vruntime(cfs_rq->min_vruntime, vruntime);
+
 #ifndef CONFIG_64BIT
 	smp_wmb();
 	cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
-- 
1.9.1