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 EBAF6EB64DA for ; Wed, 28 Jun 2023 08:10:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233856AbjF1IKL (ORCPT ); Wed, 28 Jun 2023 04:10:11 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36022 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233143AbjF1IFw (ORCPT ); Wed, 28 Jun 2023 04:05:52 -0400 Received: from out-16.mta0.migadu.com (out-16.mta0.migadu.com [IPv6:2001:41d0:1004:224b::10]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9899330C7 for ; Wed, 28 Jun 2023 01:04:36 -0700 (PDT) Message-ID: <6d86df7f-89c7-745c-c5c9-285628ef984f@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1687934643; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=X+4/1a9FExR3xrVfAss/wqROxapr4I8GSXOcb+z9OSk=; b=rRTORJCzEUMayCRlKF/guG3MGEgfdmPM+BBC7FlyLNdEMBPrDBpxY+NKzxMcFJQ7mz0aML GtruYFZw/OaAkpqSjl2ptucE47903RouzJNlcBd6XyU8Q0aMB7CzW/tt4/HpSH/Oc/ZX4j UPvAxoPyysilphSUNFN5oqcDnXPLefc= Date: Wed, 28 Jun 2023 14:43:58 +0800 MIME-Version: 1.0 Subject: Re: [PATCH 1/4] blk-mq: use percpu csd to remote complete instead of per-rq csd Content-Language: en-US To: Ming Lei Cc: axboe@kernel.dk, tj@kernel.org, hch@lst.de, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, zhouchengming@bytedance.com References: <20230627120854.971475-1-chengming.zhou@linux.dev> <20230627120854.971475-2-chengming.zhou@linux.dev> <9c038fe8-0361-c6fb-9b4e-7b74e0591ae9@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Chengming Zhou In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023/6/28 12:50, Ming Lei wrote: > On Wed, Jun 28, 2023 at 11:28:20AM +0800, Chengming Zhou wrote: >> On 2023/6/28 10:20, Ming Lei wrote: >>> On Tue, Jun 27, 2023 at 08:08:51PM +0800, chengming.zhou@linux.dev wrote: >>>> From: Chengming Zhou >>>> >>>> If request need to be completed remotely, we insert it into percpu llist, >>>> and smp_call_function_single_async() if llist is empty previously. >>>> >>>> We don't need to use per-rq csd, percpu csd is enough. And the size of >>>> struct request is decreased by 24 bytes. >>>> >>>> Signed-off-by: Chengming Zhou >>>> --- >>>> block/blk-mq.c | 12 ++++++++---- >>>> include/linux/blk-mq.h | 5 +---- >>>> 2 files changed, 9 insertions(+), 8 deletions(-) >>>> >>>> diff --git a/block/blk-mq.c b/block/blk-mq.c >>>> index decb6ab2d508..a36822479b94 100644 >>>> --- a/block/blk-mq.c >>>> +++ b/block/blk-mq.c >>>> @@ -43,6 +43,7 @@ >>>> #include "blk-ioprio.h" >>>> >>>> static DEFINE_PER_CPU(struct llist_head, blk_cpu_done); >>>> +static DEFINE_PER_CPU(struct __call_single_data, blk_cpu_csd); >>> >>> It might be better to use call_single_data, given: >>> >>> /* Use __aligned() to avoid to use 2 cache lines for 1 csd */ >>> typedef struct __call_single_data call_single_data_t >>> __aligned(sizeof(struct __call_single_data)); >>> >> >> Good, I will change to use this. >> >>>> >>>> static void blk_mq_insert_request(struct request *rq, blk_insert_t flags); >>>> static void blk_mq_request_bypass_insert(struct request *rq, >>>> @@ -1156,13 +1157,13 @@ static void blk_mq_complete_send_ipi(struct request *rq) >>>> { >>>> struct llist_head *list; >>>> unsigned int cpu; >>>> + struct __call_single_data *csd; >>>> >>>> cpu = rq->mq_ctx->cpu; >>>> list = &per_cpu(blk_cpu_done, cpu); >>>> - if (llist_add(&rq->ipi_list, list)) { >>>> - INIT_CSD(&rq->csd, __blk_mq_complete_request_remote, rq); >>>> - smp_call_function_single_async(cpu, &rq->csd); >>>> - } >>>> + csd = &per_cpu(blk_cpu_csd, cpu); >>>> + if (llist_add(&rq->ipi_list, list)) >>>> + smp_call_function_single_async(cpu, csd); >>>> } >>> >>> This way is cleaner, and looks correct, given block softirq is guaranteed to be >>> scheduled to consume the list if one new request is added to this percpu list, >>> either smp_call_function_single_async() returns -EBUSY or 0. >>> >> >> If this llist_add() see the llist is empty, the consumer function in the softirq >> on the remote CPU must have consumed the llist, so smp_call_function_single_async() >> won't return -EBUSY ? > > block softirq can be scheduled from other code path, such as blk_mq_raise_softirq() > for single queue's remote completion, where no percpu csd schedule is needed, so > two smp_call_function_single_async() could be called, and the 2nd one > may return -EBUSY. Thanks for your very clear explanation! I understand what you mean. Yes, the 2nd smp_call_function_single_async() will return -EBUSY, but it's ok since the 1st will do the right thing. > > Not mention csd_unlock() could be called after the callback returns, see > __flush_smp_call_function_queue(). Ok, CSD_TYPE_SYNC will csd_unlock() after csd_do_func() returns, our CSD_TYPE_ASYNC will csd_unlock() before csd_do_func(). > > But that is fine, if there is pending block softirq, the llist is > guaranteed to be consumed because the csd callback just raises block > softirq, and request/llist is consumed in softirq handler. > Agree, it's fine even the 2nd return -EBUSY when the 1st function is raising block softirq, our llist will be consumed in softirq handler. Thanks!