linux-kernel - [BUG] Bw accounting warning on fair-servers' parameters change

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Message-ID: <20250718113848.193139-1-yurand2000@gmail.com>
Date: Fri, 18 Jul 2025 13:38:48 +0200
From: Yuri Andriaccio <yurand2000@...il.com>
To: Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>
Cc: linux-kernel@...r.kernel.org,
	Luca Abeni <luca.abeni@...tannapisa.it>,
	Yuri Andriaccio <yuri.andriaccio@...tannapisa.it>
Subject: [BUG] Bw accounting warning on fair-servers' parameters change

Hi,

I've been lately working on fair-servers and dl_servers for some patches and
I've come across a bandwidth accounting warning on the latest tip/master (as of
2025-07-18, git sha ed0272f0675f). The warning is triggered by simply starting
the machine, mounting debugfs and then just zeroing any fair-server's runtime.

The warning:

WARNING: kernel/sched/deadline.c:266 at dl_rq_change_utilization+0x208/0x230
static inline void __sub_rq_bw(u64 dl_bw, struct dl_rq *dl_rq) {
    ...
	WARN_ON_ONCE(dl_rq->running_bw > dl_rq->this_bw);
}

Steps to reproduce:

mount -t debugfs none /sys/kernel/debug
echo 0 > /sys/kernel/debug/sched/fair_server/cpu0/runtime

It does not happen at every machine boot, but happens on most. Could it possibly
be related to some of the deadline timers?

I'll probably follow up with some more tests to better understand what is going
on but I first wanted to bring this up.

Have a nice day,
Yuri