From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id ACC72CCD185 for ; Mon, 13 Oct 2025 20:32:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: MIME-Version:References:In-Reply-To:Message-ID:Date:Subject:Cc:To:From: Reply-To:Content-Type:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=ucvmGdln0eqXccg+XKAlaJPI55ctMmMl9RgMkth7GwU=; b=Ouu9ZniF1ml+SphWK7v1rK3qAy /CYSWpe6XSYQUv/1sM+etG+c8uBFKFEu2+Kx8UuWTPIINtjqhaAmSH4xuksMEVejCoP5dkMJcIGih YzRK6igUR/dS7TpbUg+vb9aWzBpXc08HMXaqH5VYRaFLN/yv1SXBvmT1yDRERl62sT9jcL/hT0ENT z3efJe2SE1v0fYIcjiPs4Kcn2N4gpmD7DmStivAwrF20900mVWcTOEr37f5colklzaTG4uOAY32Kr gG07UAiF40kDf6o2ajX9IZNtjrGOu/Qj0I12EMACAGUn17vwU9lgE4BX2J+mzW7YtKoD7SIE4ee5q VOMGvilg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1v8PDN-0000000EMr3-2EBs; Mon, 13 Oct 2025 20:32:17 +0000 Received: from sea.source.kernel.org ([172.234.252.31]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1v8PDK-0000000EMpZ-3Xpy for linux-arm-kernel@lists.infradead.org; Mon, 13 Oct 2025 20:32:15 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 2E4584172A; Mon, 13 Oct 2025 20:32:14 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id E2B4AC116C6; Mon, 13 Oct 2025 20:32:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1760387534; bh=bjPVH71yYHRT38x+a2qhkcGYmz0Liwe3xJMHVBXj3C4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=b25AQpoE2Gaccq6xqz9Wp4kTc+E9b5qQGmXxn4aXI8OOn1B6UWNO90qdLaRbPI+WV 5aT/vU5QYvbKG49S3wjZ41D44+GQQW0ZOcp7K4hq883Qob1sALGR40UhGm0KN2PlN9 lve9n4RO7Czep7SFLzwH/NsLIsfatogAOXApPZHKKTObGY2vqfYCGn3UbQ6SZZ4CW/ jRAmfvkvLzSH2xaloDMzLSTzfa/vxerXiq24jTwgeXFCtmkcBbFweebJdJHNI5fUog vGDyVAFB2RxSJMOn45X+ySzq3gIoohBiCt8cSMxIiRZ9K3OOIGCMlH9XqW0LQd1h1e TfzyOayYLeI3A== From: Frederic Weisbecker To: LKML Cc: Frederic Weisbecker , =?UTF-8?q?Michal=20Koutn=C3=BD?= , Andrew Morton , Bjorn Helgaas , Catalin Marinas , Danilo Krummrich , "David S . Miller" , Eric Dumazet , Gabriele Monaco , Greg Kroah-Hartman , Ingo Molnar , Jakub Kicinski , Jens Axboe , Johannes Weiner , Lai Jiangshan , Marco Crivellari , Michal Hocko , Muchun Song , Paolo Abeni , Peter Zijlstra , Phil Auld , "Rafael J . Wysocki" , Roman Gushchin , Shakeel Butt , Simon Horman , Tejun Heo , Thomas Gleixner , Vlastimil Babka , Waiman Long , Will Deacon , cgroups@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-block@vger.kernel.org, linux-mm@kvack.org, linux-pci@vger.kernel.org, netdev@vger.kernel.org Subject: [PATCH 02/33] cpu: Revert "cpu/hotplug: Prevent self deadlock on CPU hot-unplug" Date: Mon, 13 Oct 2025 22:31:15 +0200 Message-ID: <20251013203146.10162-3-frederic@kernel.org> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20251013203146.10162-1-frederic@kernel.org> References: <20251013203146.10162-1-frederic@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20251013_133214_914929_FA676796 X-CRM114-Status: GOOD ( 18.19 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 1) The commit: 2b8272ff4a70 ("cpu/hotplug: Prevent self deadlock on CPU hot-unplug") was added to fix an issue where the hotplug control task (BP) was throttled between CPUHP_AP_IDLE_DEAD and CPUHP_HRTIMERS_PREPARE waiting in the hrtimer blindspot for the bandwidth callback queued in the dead CPU. 2) Later on, the commit: 38685e2a0476 ("cpu/hotplug: Don't offline the last non-isolated CPU") plugged on the target selection for the workqueue offloaded CPU down process to prevent from destroying the last CPU domain. 3) Finally: 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from outgoing CPU earlier") removed entirely the conditions for the race exposed and partially fixed in 1). The offloading of the CPU down process to a workqueue on another CPU then becomes unnecessary. But the last CPU belonging to scheduler domains must still remain online. Therefore revert the now obsolete commit 2b8272ff4a70b866106ae13c36be7ecbef5d5da2 and move the housekeeping check under the cpu_hotplug_lock write held. Since HK_TYPE_DOMAIN will include both isolcpus and cpuset isolated partition, the hotplug lock will synchronize against concurrent cpuset partition updates. Signed-off-by: Frederic Weisbecker --- kernel/cpu.c | 37 +++++++++++-------------------------- 1 file changed, 11 insertions(+), 26 deletions(-) diff --git a/kernel/cpu.c b/kernel/cpu.c index db9f6c539b28..453a806af2ee 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -1410,6 +1410,16 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen, cpus_write_lock(); + /* + * Keep at least one housekeeping cpu onlined to avoid generating + * an empty sched_domain span. + */ + if (cpumask_any_and(cpu_online_mask, + housekeeping_cpumask(HK_TYPE_DOMAIN)) >= nr_cpu_ids) { + ret = -EBUSY; + goto out; + } + cpuhp_tasks_frozen = tasks_frozen; prev_state = cpuhp_set_state(cpu, st, target); @@ -1456,22 +1466,8 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen, return ret; } -struct cpu_down_work { - unsigned int cpu; - enum cpuhp_state target; -}; - -static long __cpu_down_maps_locked(void *arg) -{ - struct cpu_down_work *work = arg; - - return _cpu_down(work->cpu, 0, work->target); -} - static int cpu_down_maps_locked(unsigned int cpu, enum cpuhp_state target) { - struct cpu_down_work work = { .cpu = cpu, .target = target, }; - /* * If the platform does not support hotplug, report it explicitly to * differentiate it from a transient offlining failure. @@ -1480,18 +1476,7 @@ static int cpu_down_maps_locked(unsigned int cpu, enum cpuhp_state target) return -EOPNOTSUPP; if (cpu_hotplug_disabled) return -EBUSY; - - /* - * Ensure that the control task does not run on the to be offlined - * CPU to prevent a deadlock against cfs_b->period_timer. - * Also keep at least one housekeeping cpu onlined to avoid generating - * an empty sched_domain span. - */ - for_each_cpu_and(cpu, cpu_online_mask, housekeeping_cpumask(HK_TYPE_DOMAIN)) { - if (cpu != work.cpu) - return work_on_cpu(cpu, __cpu_down_maps_locked, &work); - } - return -EBUSY; + return _cpu_down(cpu, 0, target); } static int cpu_down(unsigned int cpu, enum cpuhp_state target) -- 2.51.0