From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=158.69.130.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794015902; b=4tQCSvK4zzF6vIxVEZXOTxC+tmvdu/jcnTe1PNAtWvVT7HgFNXdNkL6R9HQpSt/TaEqD aKvPka3cl1KTT5zHmQkpG97ZKnWeRooHsAqRave381morq95iEC/ybQ3/5HbvM0BkMLv9 Qb6KCGJRTvtvI8+2qYIcWAgCAc37bmq0ZiPrbtJ1bZ6V+qR7t0eIdrSrKb5IDhCBY5gvd eJT8hS7h9+EQ1oNTMD9ZqyaTrMIFd6TvRaWmv2i9mLufSOSdC1MBdsk5Q1rp7fpRLFtqB RUOORUgyWOPk5KOAHR7YJcOILOT1+GsNpB+m9FNlUcNwFKdHeB//b8RethX0uR8Kb5w== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794015902; h=DMARC-Filter:DKIM-Signature:Received:From:To:Cc:Subject:Date: Message-Id:X-Mailer:In-Reply-To:References:MIME-Version: Content-Transfer-Encoding; bh=ufKD3yo0IU8O9h+5AOdnMqxb+tYGqKSNZQWh8YpfE7s=; b=5E8MqIykmcv9gXw1p4pxn2STGWSc7RsTVCzMiT7/XIHRYeEQCfYEUem2oJyN4eQYXUWR ejy+doP1s/zCdts+gGyTyTKCVcc0nnr5Zcf5yAzXW6iPy7FkIs4kSN96JUIBBAK+aw9+k +dh2G8q+wQB0F45+fBU3CEbuj3V96L013pfnCeU6U0NvTHq9mUe0xCjldleJ7mmUeMBAM qfBUavmd+tq8ejAg0clAtCOP9ASIqEb8xtq64PJpA66bhGeUEwiHuyduu8c0pUmXwys4y EWPJjSufPbv05sWO3MXDDnxc3nMxSkIDFOGHUcI5ECZpTo3zBHPLqi9ytdGX0QazxFw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=efficios.com; spf=pass smtp.mailfrom=efficios.com; dkim=pass header.d=efficios.com header.i=@efficios.com header.a=rsa-sha256 header.s=smtpout1 header.b=m0KNl5gN; arc=none smtp.remote-ip=158.69.130.18 Received: from smtpout.efficios.com (smtpout.efficios.com [158.69.130.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5BDE7606D813; Sat, 7 Nov 2026 01:45:01 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 5BDE7606D813 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=efficios.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=efficios.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=efficios.com header.i=@efficios.com header.a=rsa-sha256 header.s=smtpout1 header.b=m0KNl5gN DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficios.com; s=smtpout1; t=1794015900; bh=ufKD3yo0IU8O9h+5AOdnMqxb+tYGqKSNZQWh8YpfE7s=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=m0KNl5gNgQVilcQ6DGQivWxnzyaz3mViWGDftwT4aV0FlFigFFs0yoD7YuAcZEOrL rtdrgJuOnxS5jJwwTOjvw+eA9xIK7fRNOA5G3bq3z2U6Ho2kKfgHHStCHNE5UyOoM2 wLr+rEC9hP8ZnQKMvNAnykBz/rilxxMoOGj1KgE2lli6hi6IsS3j7MW208MjdGJnnc i35nWn09f2/NPExXpRxHTESHhWVD5U/gLMF/Vwue5878881XAtVLRKfPC5vzImJRll n2IsLUX8Hj06MB0ZIIZpZvOG/Xj88Bc+cSoKmGXgZo/Fhzy9pGCAG23qon8DweWPr6 Nmk3eu8dfkSmQ== Received: from thinkos.internal.efficios.com (unknown [64.58.40.251]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4jLzPm0j2vzLgS; Fri, 06 Nov 2026 20:45:00 -0500 (EST) From: Mathieu Desnoyers To: juri.lelli@redhat.com, peterz@infradead.org, mingo@redhat.com, vincent.guittot@linaro.org Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers , tglx@kernel.org, will@kernel.org, boqun@kernel.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, kprateek.nayak@amd.com, luca.abeni@santannapisa.it, mathieu.poirier@linaro.org, longman@redhat.com, ridong.chen@linux.dev, tj@kernel.org, hannes@cmpxchg.org, mkoutny@suse.com, shuah@kernel.org, bigeasy@linutronix.de, clrkwllms@kernel.org, parri.andrea@gmail.com, m.szyprowski@samsung.com, kayracizmeci@gmail.com, weihengqiuu@gmail.com, cgroups@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-rt-devel@lists.linux.dev, stable@vger.kernel.org Subject: [PATCH v4 05/26] sched/deadline: Do not divide by zero active CPUs of a root domain Date: Fri, 6 Nov 2026 20:44:30 -0500 Message-Id: <20261107014451.1487825-6-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20261107014451.1487825-1-mathieu.desnoyers@efficios.com> References: <20261107014451.1487825-1-mathieu.desnoyers@efficios.com> 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-Transfer-Encoding: 8bit __dl_sub() and __dl_add() spread a change of a root domain's total_bw over the extra_bw of its active CPUs, dividing it by their number, dl_bw_cpus(). That number is 0 for a root domain whose CPUs all went offline, such as the default root domain, which holds the offline CPUs: the division traps. A debugfs write of a fair server's runtime or period divides there while its CPU goes offline or comes online: the CPU sits in the default root domain from the rebuild in sched_cpu_deactivate() until it is offline, and from its online until sched_cpu_activate(), and commit 4043f5498416 ("sched/deadline: Reject debugfs dl_server writes for offline CPUs") refuses the write only once the CPU is offline. A write that leaves the server's bandwidth as it is passes admission there. Writing a fair server's own runtime back in a loop while its CPU goes offline and online oopses on the first round, in 8-CPU VMs of v7.4-rc1: Oops: divide error: 0000 [#1] SMP NOPTI RIP: 0010:dl_server_apply_params+0x335/0x380 Call Trace: sched_server_write_common.isra.0+0x1c0/0x260 full_proxy_write+0x58/0x90 vfs_write+0xcb/0x550 inactive_task_timer() can divide too, found by reading: it releases the bandwidth of a task that left SCHED_DEADLINE, or died, from the root domain of its CPU, which may have gone offline since. With admission control off, so can sched_setattr(SCHED_DEADLINE) of a task asleep on an offline CPU. Refuse the write while the runqueue is offline, as once the CPU is: the server's bandwidth is not counted then, and the rebuild at the CPU's activation counts what the server has. rq->online is set under the runqueue lock by the attach to a root domain that counts the CPU, and cleared before the rebuild that drops it, where cpu_active() is set before the attach and would be read without cpus_read_lock(). With no active CPU there is no extra_bw to update either: change total_bw alone, for the paths that have no write to refuse. 20 rounds of the reproducer leave no warning then, its writes refused with EBUSY in the window. Fixes: daec57983670 ("sched/deadline: Reclaim bandwidth not used by dl tasks") Cc: stable@vger.kernel.org Signed-off-by: Mathieu Desnoyers --- kernel/sched/deadline.c | 11 +++++++++-- kernel/sched/debug.c | 9 ++++++++- 2 files changed, 17 insertions(+), 3 deletions(-) diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index dfae08625342..00983487bff3 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -189,18 +189,25 @@ void __dl_update(struct dl_bw *dl_b, s64 bw) } } +/* + * @cpus is the number of active CPUs of the root domain, whose extra_bw + * shares the change: none once they all went offline, and total_bw alone + * changes then. + */ static inline void __dl_sub(struct dl_bw *dl_b, u64 tsk_bw, int cpus) { dl_b->total_bw -= tsk_bw; - __dl_update(dl_b, (s32)tsk_bw / cpus); + if (cpus) + __dl_update(dl_b, (s32)tsk_bw / cpus); } static inline void __dl_add(struct dl_bw *dl_b, u64 tsk_bw, int cpus) { dl_b->total_bw += tsk_bw; - __dl_update(dl_b, -((s32)tsk_bw / cpus)); + if (cpus) + __dl_update(dl_b, -((s32)tsk_bw / cpus)); } static inline bool diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c index e6a3b516c703..5f8303630296 100644 --- a/kernel/sched/debug.c +++ b/kernel/sched/debug.c @@ -410,7 +410,14 @@ static ssize_t sched_server_write_common(struct file *filp, const char __user *u return -EINVAL; } - if (!cpu_online(cpu_of(rq))) + /* + * Nor while its runqueue is offline, the CPU going offline or + * coming online: the server's bandwidth is not counted in a + * root domain then, which has no active CPU to spread a + * change over. rq->online follows the attach to a root domain + * under this lock; cpu_active() is set before it. + */ + if (!rq->online) return -EBUSY; update_rq_clock(rq); -- 2.47.3