[tip: sched/urgent] sched/debug: Fix avg_vruntime() usage

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     e08d007f9d813616ce7093600bc4fdb9c9d81d89
Gitweb:        https://git.kernel.org/tip/e08d007f9d813616ce7093600bc4fdb9c9d81d89
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Wed, 01 Apr 2026 15:20:21 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 02 Apr 2026 13:42:43 +02:00

sched/debug: Fix avg_vruntime() usage

John reported that stress-ng-yield could make his machine unhappy and
managed to bisect it to commit b3d99f43c72b ("sched/fair: Fix
zero_vruntime tracking").

The commit in question changes avg_vruntime() from a function that is
a pure reader, to a function that updates variables. This turns an
unlocked sched/debug usage of this function from a minor mistake into
a data corruptor.

Fixes: af4cf40470c2 ("sched/fair: Add cfs_rq::avg_vruntime")
Fixes: b3d99f43c72b ("sched/fair: Fix zero_vruntime tracking")
Reported-by: John Stultz <jstultz@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Tested-by: John Stultz <jstultz@xxxxxxxxxx>
Link: https://patch.msgid.link/20260401132355.196370805@xxxxxxxxxxxxx
---
 kernel/sched/debug.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index b24f40f..15bf45b 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -902,6 +902,7 @@ static void print_rq(struct seq_file *m, struct rq *rq, int rq_cpu)
 void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
 {
 	s64 left_vruntime = -1, zero_vruntime, right_vruntime = -1, left_deadline = -1, spread;
+	u64 avruntime;
 	struct sched_entity *last, *first, *root;
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long flags;
@@ -925,6 +926,7 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
 	if (last)
 		right_vruntime = last->vruntime;
 	zero_vruntime = cfs_rq->zero_vruntime;
+	avruntime = avg_vruntime(cfs_rq);
 	raw_spin_rq_unlock_irqrestore(rq, flags);
 
 	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "left_deadline",
@@ -934,7 +936,7 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
 	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "zero_vruntime",
 			SPLIT_NS(zero_vruntime));
 	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "avg_vruntime",
-			SPLIT_NS(avg_vruntime(cfs_rq)));
+			SPLIT_NS(avruntime));
 	SEQ_printf(m, "  .%-30s: %Ld.%06ld\n", "right_vruntime",
 			SPLIT_NS(right_vruntime));
 	spread = right_vruntime - left_vruntime;