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Tue, 26 May 2026 05:07:55 -0700 (PDT) Received: from jlelli-thinkpadt14gen4.remote.csb ([151.29.44.165]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-45ed90f8978sm2741326f8f.7.2026.05.26.05.07.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 26 May 2026 05:07:54 -0700 (PDT) Date: Tue, 26 May 2026 14:07:53 +0200 From: Juri Lelli To: Andrea Righi Cc: Ingo Molnar , Peter Zijlstra , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , K Prateek Nayak , linux-kernel@vger.kernel.org, Sashiko Subject: Re: [PATCH] sched/deadline: Reject debugfs dl_server writes for offline CPUs Message-ID: References: <20260526100502.575774-1-arighi@nvidia.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260526100502.575774-1-arighi@nvidia.com> Hi Andrea, On 26/05/26 12:05, Andrea Righi wrote: > Writing runtime or period via the per-CPU dl_server debugfs files > (/sys/kernel/debug/sched/{fair,ext}_server/cpu*/{runtime,period}) on an > offline CPU can trigger two distinct kernel issues: > > 1) Divide-by-zero in dl_server_apply_params(): > > Oops: divide error: 0000 [#1] SMP NOPTI > RIP: 0010:dl_server_apply_params+0x239/0x3a0 > Call Trace: > sched_server_write_common.isra.0+0x21a/0x3c0 > full_proxy_write+0x78/0xd0 > vfs_write+0xe7/0x6e0 > > Both __dl_sub() and __dl_add() divide by cpus internally, which can be > 0 once the CPU has been removed from any active root-domain span (this > has been latent since the debugfs interface was introduced). > > 2) WARN_ON_ONCE in dl_server_start(): > > WARNING: kernel/sched/deadline.c:1805 at dl_server_start+0x232/0x270 > > Commit ee6e44dfe6e5 ("sched/deadline: Stop dl_server before CPU goes > offline") added this check to catch enqueueing the server on an > offline rq. > > There's no meaningful semantics for re-configuring the per-CPU dl_server > bandwidth while the CPU is offline, so simply reject the write with > -EBUSY so userspace gets a clear error. > > Reported-by: Sashiko > Closes: https://lore.kernel.org/all/20260526092228.3B6891F00A3A@smtp.kernel.org/ > Fixes: d741f297bcea ("sched/fair: Fair server interface") > Signed-off-by: Andrea Righi > --- > kernel/sched/debug.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c > index ed3a0d65da0ca..e57ad8c78a60e 100644 > --- a/kernel/sched/debug.c > +++ b/kernel/sched/debug.c > @@ -415,6 +415,9 @@ static ssize_t sched_server_write_common(struct file *filp, const char __user *u > return -EINVAL; > } > > + if (!cpu_online(cpu_of(rq))) > + return -EBUSY; > + > update_rq_clock(rq); > dl_server_stop(dl_se); > retval = dl_server_apply_params(dl_se, runtime, period, 0); I was looking at Sashiko findings and wondered what to do about this as well. I think what you are proposing should be fine, unless for some reason one wants to tweak dl-server parameters before swithcing a CPU on. but since hotplug it's a disruptive operation already, I would say imposing to make such a change after CPU is online should be ok (and simpler to get right from a bandwidth accounting pov). Reviewed-by: Juri Lelli Thanks, Juri