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Fri, 13 Nov 2026 06:48:38 -0800 (PST) X-Received: by 2002:a05:600c:3acf:b0:4ba:4395:560f with SMTP id 5b1f17b1804b1-4bb55a70e39mr62401905e9.8.1794581317838; Fri, 13 Nov 2026 06:48:37 -0800 (PST) Received: from jlelli-thinkpadt14gen4.remote.csb ([151.29.158.90]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4bb55ace535sm68040475e9.6.2026.11.13.06.48.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 13 Nov 2026 06:48:35 -0800 (PST) Date: Fri, 13 Nov 2026 15:48:32 +0100 From: Juri Lelli To: Sebastian Andrzej Siewior Cc: linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, rcu@vger.kernel.org, linux-doc@vger.kernel.org, Jonathan Corbet , Shuah Khan , Randy Dunlap , Ingo Molnar , Peter Zijlstra , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , K Prateek Nayak , luca abeni , Yuri Andriaccio , Yuri Andriaccio , "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Davidlohr Bueso , Tejun Heo , Johannes Weiner , Michal =?iso-8859-1?Q?Koutn=FD?= , Qais Yousef , John Ogness Subject: Re: [PATCH v3 1/8] sched/deadline: Reject debugfs dl_server writes for not-active CPUs. Message-ID: References: <20261113-sched_dl-v3-0-a76ffa1cdeee@linutronix.de> <20261113-sched_dl-v3-1-a76ffa1cdeee@linutronix.de> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261113-sched_dl-v3-1-a76ffa1cdeee@linutronix.de> Hello, On 13/11/26 11:50, Sebastian Andrzej Siewior wrote: > sched_server_write_common() checks if the CPU is online before allowing > fair_server updates. This is important because updating the fair_server > runtime/ period will use dl_bw_cpus() will return only the number of > CPUs which are also marked active. > > The CPU is marked active as the last step during CPU-hotplug. If the CPU > has been marked "online" but not yet "active" or is stuck between those > two states then any update of period/ runtime will lead to a divide-by-0 > error. > > Let sched_server_write_common() check of active CPUs instead of online. > Additonaly, let dl_server_apply_params() return an error if dl_bw_cpus() > return 0 CPUs to avoid the divide error. > > Fixes: 4043f54984161 ("sched/deadline: Reject debugfs dl_server writes for offline CPUs") > Signed-off-by: Sebastian Andrzej Siewior > --- > kernel/sched/deadline.c | 3 +++ > kernel/sched/debug.c | 2 +- > 2 files changed, 4 insertions(+), 1 deletion(-) > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c > index dfae08625342e..10beb54281881 100644 > --- a/kernel/sched/deadline.c > +++ b/kernel/sched/deadline.c > @@ -1920,6 +1920,9 @@ int dl_server_apply_params(struct sched_dl_entity *dl_se, u64 runtime, u64 perio > cpus = dl_bw_cpus(cpu); > cap = dl_bw_capacity(cpu); > > + if (!cpus) > + return -EBUSY; > + > if (__dl_overflow(dl_b, cap, old_bw, new_bw)) > return -EBUSY; > > diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c > index e6a3b516c703b..1545139e52443 100644 > --- a/kernel/sched/debug.c > +++ b/kernel/sched/debug.c > @@ -410,7 +410,7 @@ static ssize_t sched_server_write_common(struct file *filp, const char __user *u > return -EINVAL; > } > > - if (!cpu_online(cpu_of(rq))) > + if (!cpu_active(cpu_of(rq))) > return -EBUSY; Mathieu's approach [1] looks safer to me. sched_cpu_deactivate() calls set_cpu_active(false) without rq->lock, so a write that passed the cpu_active() check can still be in flight until sched_set_rq_offline() (which the !cpus check then has to cover). On the online side, set_cpu_active(true) runs before the domains rebuild, so the check lets the write through against the def rd's accounting. rq->online is only changed under the rq->lock this write already holds. Thanks, Juri 1 - 20261108185406.1549770-6-mathieu.desnoyers@efficios.com