From patchwork Thu Aug 31 09:37:54 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ming Lei X-Patchwork-Id: 13371175 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 2E400C83F10 for ; Thu, 31 Aug 2023 09:38:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231459AbjHaJi4 (ORCPT ); Thu, 31 Aug 2023 05:38:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46846 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230428AbjHaJi4 (ORCPT ); Thu, 31 Aug 2023 05:38:56 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 50A6A194 for ; Thu, 31 Aug 2023 02:38:07 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1693474686; 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=pIHkr0l2kEulrVce5uaSBSAoxxMZwAnZ1GdIhmQZ2hI=; b=h4Rw6WVct884FOtiMCjSlxg8phvuiksBY/UTHhhWCGh4lOlII0kcMwiSjORwn7BxVEeZah obbxojbcOPATdPpUNcOTckUO8oJOhu6X9pE6d7D0NPzRv9WkXLHQtWWU/AKcCbDT6P9938 BH0bnlkyJ4QAJp424lPMjCTn+affKaY= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-511-LnRHs2JGOduXWRKhYf_VWg-1; Thu, 31 Aug 2023 05:38:02 -0400 X-MC-Unique: LnRHs2JGOduXWRKhYf_VWg-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 84D7985CBE0; Thu, 31 Aug 2023 09:38:00 +0000 (UTC) Received: from localhost (unknown [10.72.120.4]) by smtp.corp.redhat.com (Postfix) with ESMTP id 3F65C1121315; Thu, 31 Aug 2023 09:37:58 +0000 (UTC) From: Ming Lei To: Jens Axboe , 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 Subject: [PATCH V4] lib/group_cpus.c: avoid to acquire cpu hotplug lock in group_cpus_evenly Date: Thu, 31 Aug 2023 17:37:54 +0800 Message-Id: <20230831093754.2322955-1-ming.lei@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.3 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 Signed-off-by: Ming Lei Reviewed-by: Jens Axboe --- V4: - annotate lockess read of cpu_present_mask with data_race() - document why lockless read of cpu_present_mask is correct for group_cpus_evenly, as suggested by Thomas V3: - reuse `npresmsk`, and avoid to allocate new variable, suggested by Chengming Zhou V2: - fix "Cc: block list" - add tested-by tag 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);