From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="HT4wAfcu" 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 7292ED5C for ; Wed, 6 Dec 2023 17:11:27 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1701911486; 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=ExJdtbKmNQpxsox6HxA6icn3NiLLvZaWuFWfZqOZ2U4=; b=HT4wAfcuERSkvkih3Ctju5LW7uVQhcaEZIjnKLwU/vSVwypD6aQ52bETfJbC8Z2PN28z4j vOJpb2tqTzRukV0qr0032D+HC5MQwrfm1LJuPfX+znz2mab6aRPYtqtF2uqgADcjMiBKUn T00sw3tX/M540SCe0ii1UYkBDLYaOfk= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-609-MjrAbh1mMma7D6M1fsK8pQ-1; Wed, 06 Dec 2023 20:11:21 -0500 X-MC-Unique: MjrAbh1mMma7D6M1fsK8pQ-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id AF34E283780C; Thu, 7 Dec 2023 01:11:20 +0000 (UTC) Received: from fedora (unknown [10.72.120.12]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 238F7492BC6; Thu, 7 Dec 2023 01:11:13 +0000 (UTC) Date: Thu, 7 Dec 2023 09:11:09 +0800 From: Ming Lei To: Yury Norov Cc: Thomas Gleixner , Andrew Morton , linux-kernel@vger.kernel.org, Keith Busch , linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, Yi Zhang , Guangwu Zhang , Chengming Zhou , Jens Axboe Subject: Re: [PATCH V4 resend] lib/group_cpus.c: avoid to acquire cpu hotplug lock in group_cpus_evenly Message-ID: References: <20231120083559.285174-1-ming.lei@redhat.com> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.9 On Wed, Dec 06, 2023 at 04:41:44PM -0800, Yury Norov wrote: > Hi Ming, > > On Mon, Nov 20, 2023 at 04:35:59PM +0800, 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 > > 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)); > > Now that you initialize the npresmsk explicitly, you can allocate it > using alloc_cpumask_var(). Indeed, but this way is actually before this patch, and not related with this fix. > > The same actually holds for nmsk too, and even before this patch. Maybe > fix it in a separate prepending patch? Yeah, 'nmsk' is similar with 'npresmsk', and it is not fix, just one optimization. group_cpus_evenly() is only run in slow path, so this kind of micro-optimization is not urgent and should be done in standalone patch, and even we can live with it. > > > + > > /* 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); > > The first thing the helper does is checking if nprepmask is empty. > cpumask_andnot() returns false in that case. So, assuming that present > cpumask in the previous call can't be empty, we can save few cycles if > drop corresponding check in the helper and do like this: > > if (cpumask_andnot(npresmsk, cpu_possible_mask, npresmsk) == 0) { > nr_others = 0; > goto fail_build_affinity; > } > > ret = __group_cpus_evenly(curgrp, numgrps, node_to_cpumask, > npresmsk, nmsk, masks); > > Although, it's not related to this patch directly. So, if you fix > zalloc_cpumask_var(), the patch looks good to me. I'd rather not make things complicated, as mentioned this API is only run in slow path. > > Reviewed-by: Yury Norov Thanks for the review! Thanks, Ming