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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C9D56C2BB3F for ; Mon, 20 Nov 2023 20:02:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229497AbjKTUCP (ORCPT ); Mon, 20 Nov 2023 15:02:15 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44268 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229689AbjKTUCO (ORCPT ); Mon, 20 Nov 2023 15:02:14 -0500 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CE634BE for ; Mon, 20 Nov 2023 12:02:10 -0800 (PST) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6B513C433C8; Mon, 20 Nov 2023 20:02:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1700510530; bh=4R2Hqef/lBEVXTL7PTv3MFyycRKQ23T9rDbyXg7dUXs=; h=Date:To:From:Subject:From; b=ek0RnKUToZ/tBENJM2uRk7F07WFCEwRFvTABesZxFf0JhgEu7bHojiOxADln4pWRI A0il7XjEEOsRZ4u2mEim/i8NsPR4tndihLrZgbe37mvqmPsFeCn097en/9z003XVi5 Z0uQy4Vhew7N3XIsmWxysQ3Hzz0znSHSNRH+AOA0= Date: Mon, 20 Nov 2023 12:02:09 -0800 To: mm-commits@vger.kernel.org, zhouchengming@bytedance.com, yi.zhang@redhat.com, kbusch@kernel.org, guazhang@redhat.com, axboe@kernel.dk, ming.lei@redhat.com, akpm@linux-foundation.org From: Andrew Morton Subject: + lib-group_cpusc-avoid-to-acquire-cpu-hotplug-lock-in-group_cpus_evenly.patch added to mm-hotfixes-unstable branch Message-Id: <20231120200210.6B513C433C8@smtp.kernel.org> Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org The patch titled Subject: lib/group_cpus.c: avoid acquiring cpu hotplug lock in group_cpus_evenly has been added to the -mm mm-hotfixes-unstable branch. Its filename is lib-group_cpusc-avoid-to-acquire-cpu-hotplug-lock-in-group_cpus_evenly.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/lib-group_cpusc-avoid-to-acquire-cpu-hotplug-lock-in-group_cpus_evenly.patch This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days ------------------------------------------------------ From: Ming Lei Subject: lib/group_cpus.c: avoid acquiring cpu hotplug lock in group_cpus_evenly Date: Mon, 20 Nov 2023 16:35:59 +0800 group_cpus_evenly() could be part of storage driver's error handler, such as nvme driver, when may happen during CPU hotplug, in which storage queue has to drain its pending IOs because all CPUs associated with the queue are offline and the queue is becoming inactive. And handling IO needs error handler to provide forward progress. Then deadlock is caused: 1) inside CPU hotplug handler, CPU hotplug lock is held, and blk-mq's handler is waiting for inflight IO 2) error handler is waiting for CPU hotplug lock 3) inflight IO can't be completed in blk-mq's CPU hotplug handler because error handling can't provide forward progress. Solve the deadlock by not holding CPU hotplug lock in group_cpus_evenly(), in which two stage spreads are taken: 1) the 1st stage is over all present CPUs; 2) the end stage is over all other CPUs. Turns out the two stage spread just needs consistent 'cpu_present_mask', and remove the CPU hotplug lock by storing it into one local cache. This way doesn't change correctness, because all CPUs are still covered. Link: https://lkml.kernel.org/r/20231120083559.285174-1-ming.lei@redhat.com Signed-off-by: Ming Lei Reported-by: Yi Zhang Reported-by: Guangwu Zhang Tested-by: Guangwu Zhang Reviewed-by: Chengming Zhou Reviewed-by: Jens Axboe Cc: Keith Busch Signed-off-by: Andrew Morton --- lib/group_cpus.c | 22 ++++++++++++++++------ 1 file changed, 16 insertions(+), 6 deletions(-) --- a/lib/group_cpus.c~lib-group_cpusc-avoid-to-acquire-cpu-hotplug-lock-in-group_cpus_evenly +++ a/lib/group_cpus.c @@ -366,13 +366,25 @@ struct cpumask *group_cpus_evenly(unsign if (!masks) goto fail_node_to_cpumask; - /* Stabilize the cpumasks */ - cpus_read_lock(); build_node_to_cpumask(node_to_cpumask); + /* + * Make a local cache of 'cpu_present_mask', so the two stages + * spread can observe consistent 'cpu_present_mask' without holding + * cpu hotplug lock, then we can reduce deadlock risk with cpu + * hotplug code. + * + * Here CPU hotplug may happen when reading `cpu_present_mask`, and + * we can live with the case because it only affects that hotplug + * CPU is handled in the 1st or 2nd stage, and either way is correct + * from API user viewpoint since 2-stage spread is sort of + * optimization. + */ + cpumask_copy(npresmsk, data_race(cpu_present_mask)); + /* grouping present CPUs first */ ret = __group_cpus_evenly(curgrp, numgrps, node_to_cpumask, - cpu_present_mask, nmsk, masks); + npresmsk, nmsk, masks); if (ret < 0) goto fail_build_affinity; nr_present = ret; @@ -387,15 +399,13 @@ struct cpumask *group_cpus_evenly(unsign curgrp = 0; else curgrp = nr_present; - cpumask_andnot(npresmsk, cpu_possible_mask, cpu_present_mask); + cpumask_andnot(npresmsk, cpu_possible_mask, npresmsk); ret = __group_cpus_evenly(curgrp, numgrps, node_to_cpumask, npresmsk, nmsk, masks); if (ret >= 0) nr_others = ret; fail_build_affinity: - cpus_read_unlock(); - if (ret >= 0) WARN_ON(nr_present + nr_others < numgrps); _ Patches currently in -mm which might be from ming.lei@redhat.com are lib-group_cpusc-avoid-to-acquire-cpu-hotplug-lock-in-group_cpus_evenly.patch