From patchwork Fri Sep 22 08:04:25 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ming Lei X-Patchwork-Id: 13395330 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 5865DCD4F2B for ; Fri, 22 Sep 2023 08:16:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232558AbjIVIQj (ORCPT ); Fri, 22 Sep 2023 04:16:39 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46620 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232753AbjIVIQ0 (ORCPT ); Fri, 22 Sep 2023 04:16:26 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CFA0410C9 for ; Fri, 22 Sep 2023 01:04:36 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1695369876; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=CBLqoMyIVireLlLreZE4y9YWUgoFtH3j+ZLwtIAR6bY=; b=OZg+BjedXGezaJGvSswT2cR2z4Wvi+UaVVtrltDAFYnsTdpHgZ9f8XspLfva5TjeymwK/5 +AGwRakaTSyR0v0klQ3MNnOj857VtM+XieAEr12c1LvAkgqmxAI0YHFYm5JOaUJAJ0w8De kBnr40Rrg1pzVhlmMNyTa0stLjR3XUM= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-414-movzolr0O5q7EvlVO3dTTQ-1; Fri, 22 Sep 2023 04:04:32 -0400 X-MC-Unique: movzolr0O5q7EvlVO3dTTQ-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 5C4131C06E0B; Fri, 22 Sep 2023 08:04:32 +0000 (UTC) Received: from localhost (unknown [10.72.120.3]) by smtp.corp.redhat.com (Postfix) with ESMTP id 190AF40C2064; Fri, 22 Sep 2023 08:04:30 +0000 (UTC) From: Ming Lei To: Thomas Gleixner Cc: linux-kernel@vger.kernel.org, Ming Lei , Keith Busch , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, Yi Zhang , Guangwu Zhang , Chengming Zhou , Jens Axboe Subject: [PATCH V4 resend] lib/group_cpus.c: avoid to acquire cpu hotplug lock in group_cpus_evenly Date: Fri, 22 Sep 2023 16:04:25 +0800 Message-ID: <20230922080425.2745715-1-ming.lei@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.1 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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 dead lock 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. Cc: Keith Busch Cc: linux-nvme@lists.infradead.org Cc: linux-block@vger.kernel.org Reported-by: Yi Zhang Reported-by: Guangwu Zhang Tested-by: Guangwu Zhang Reviewed-by: Chengming Zhou Reviewed-by: Jens Axboe Signed-off-by: Ming Lei --- lib/group_cpus.c | 22 ++++++++++++++++------ 1 file changed, 16 insertions(+), 6 deletions(-) diff --git a/lib/group_cpus.c b/lib/group_cpus.c index aa3f6815bb12..ee272c4cefcc 100644 --- a/lib/group_cpus.c +++ b/lib/group_cpus.c @@ -366,13 +366,25 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps) 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(unsigned int numgrps) 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);