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@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: John Stultz <jstultz@google.com>
Link: https://patch.msgid.link/20260401132355.196370805@infradead.org

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index b24f40f05019a8581b02d1f926d5b6cbca067f08..15bf45b6f912412647f8c8a4b6593f2dfe22486e 100644 (file)
--- 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;