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 D6B3AC71157 for ; Fri, 18 Aug 2023 14:00:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1377572AbjHROA0 (ORCPT ); Fri, 18 Aug 2023 10:00:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39188 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1377578AbjHROAA (ORCPT ); Fri, 18 Aug 2023 10:00:00 -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 5979F421D for ; Fri, 18 Aug 2023 06:58:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1692367133; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=pOUfdHMnG05j5uCs4vC0Zp9XVakVRZi3j6poFeSIlGQ=; b=bdduu4N6OxAcVt7Mp3mEW0avIoWc6L80tq9kWZw50/Y8RkK83OoY575X+KiazLrbU4Nf1Q ae2ddoGevk/yXSAlhMThWurCnmZZGncZJUZBNlMxHMtra2e04YOcsG1IPp8YOSxfyGN1t0 595wLYG7ZA3kwJMozxPLFN/4ZxNZnkg= Received: from mimecast-mx02.redhat.com (66.187.233.73 [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-519-sMLec9EYM6SvPNB4yFmYgg-1; Fri, 18 Aug 2023 09:58:48 -0400 X-MC-Unique: sMLec9EYM6SvPNB4yFmYgg-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 784173811F4E; Fri, 18 Aug 2023 13:58:47 +0000 (UTC) Received: from fedora (unknown [10.72.120.3]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C93482026D68; Fri, 18 Aug 2023 13:58:41 +0000 (UTC) Date: Fri, 18 Aug 2023 21:58:36 +0800 From: Ming Lei To: Chengming Zhou Cc: Jens Axboe , Thomas Gleixner , linux-kernel@vger.kernel.org, Keith Busch , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, Yi Zhang , Guangwu Zhang Subject: Re: [PATCH V2] lib/group_cpus.c: avoid to acquire cpu hotplug lock in group_cpus_evenly Message-ID: References: <20230818015244.1176929-1-ming.lei@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.4 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 18, 2023 at 02:59:13PM +0800, Chengming Zhou wrote: > Hi, > > On 2023/8/18 09:52, Ming Lei wrote: > > 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 > > Signed-off-by: Ming Lei > > --- > > 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..15006e79196f 100644 > > --- a/lib/group_cpus.c > > +++ b/lib/group_cpus.c > > @@ -348,6 +348,7 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps) > > { > > unsigned int curgrp = 0, nr_present = 0, nr_others = 0; > > cpumask_var_t *node_to_cpumask; > > + cpumask_var_t local_cpu_present_mask; > > cpumask_var_t nmsk, npresmsk; > > int ret = -ENOMEM; > > struct cpumask *masks = NULL; > > @@ -355,6 +356,16 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps) > > if (!zalloc_cpumask_var(&nmsk, GFP_KERNEL)) > > return NULL; > > > > + if (!zalloc_cpumask_var(&local_cpu_present_mask, GFP_KERNEL)) > > + goto fail_local_pres_mask; > > + > > + /* > > + * Make a local cache of 'cpu_present_mask', so the two stages > > + * spread can observe consistent 'cpu_present_mask' without holding > > + * cpu hotplug lock. > > + */ > > + cpumask_copy(local_cpu_present_mask, cpu_present_mask); > > + > > Maybe we can reuse npresmsk instead of allocating another cpumask? > In the first stage: npresmsk = cpu_present_mask > In the second stage: npresmsk = cpu_possible_mask & ~npresmsk Good idea! Thanks, Ming