Commit e08d007f authored by Peter Zijlstra's avatar Peter Zijlstra
Browse files

sched/debug: Fix avg_vruntime() usage 



John reported that stress-ng-yield could make his machine unhappy and
managed to bisect it to commit b3d99f43 ("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: af4cf404 ("sched/fair: Add cfs_rq::avg_vruntime")
Fixes: b3d99f43 ("sched/fair: Fix zero_vruntime tracking")
Reported-by: default avatarJohn Stultz <jstultz@google.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarVincent Guittot <vincent.guittot@linaro.org>
Tested-by: default avatarK Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: default avatarJohn Stultz <jstultz@google.com>
Link: https://patch.msgid.link/20260401132355.196370805@infradead.org
parent 1319ea57

kernel/sched/debug.c

+3 −1
Original line number Diff line number Diff line
@@ -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;