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 X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B4255C432C0 for ; Wed, 27 Nov 2019 14:44:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 931E5206F0 for ; Wed, 27 Nov 2019 14:44:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726822AbfK0Ooj (ORCPT ); Wed, 27 Nov 2019 09:44:39 -0500 Received: from lhrrgout.huawei.com ([185.176.76.210]:2127 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726514AbfK0Ooj (ORCPT ); Wed, 27 Nov 2019 09:44:39 -0500 Received: from lhreml706-cah.china.huawei.com (unknown [172.18.7.107]) by Forcepoint Email with ESMTP id B8D57EE023C4EB3F9569; Wed, 27 Nov 2019 14:44:37 +0000 (GMT) Received: from lhreml724-chm.china.huawei.com (10.201.108.75) by lhreml706-cah.china.huawei.com (10.201.108.47) with Microsoft SMTP Server (TLS) id 14.3.408.0; Wed, 27 Nov 2019 14:44:37 +0000 Received: from [127.0.0.1] (10.202.226.46) by lhreml724-chm.china.huawei.com (10.201.108.75) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.1713.5; Wed, 27 Nov 2019 14:44:37 +0000 Subject: Re: [PATCH 3/8] blk-mq: Use a pointer for sbitmap To: Jens Axboe , Hannes Reinecke , "Martin K. Petersen" CC: Christoph Hellwig , James Bottomley , Ming Lei , "Bart van Assche" , , References: <20191126091416.20052-1-hare@suse.de> <20191126091416.20052-4-hare@suse.de> <8f0522ee-2a81-c2ae-d111-3ff89ee6f93e@kernel.dk> <62838bca-cd3c-fccf-767c-76d8bea12324@huawei.com> <00a6d920-1855-c861-caa3-e845dcbe1fd8@kernel.dk> <9290eb7f-8d0b-8012-f9a4-a49c068def1b@kernel.dk> <157f3e58-1d16-cc6b-52aa-15a6e1ac828a@huawei.com> <1add0896-4867-12c5-4507-76526c27fb56@kernel.dk> <4a780199-7997-b677-b184-411afdeabba5@huawei.com> <5bc7b976-845c-92ec-6ccc-8e43237313bc@kernel.dk> From: John Garry Message-ID: Date: Wed, 27 Nov 2019 14:44:35 +0000 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.1.2 MIME-Version: 1.0 In-Reply-To: <5bc7b976-845c-92ec-6ccc-8e43237313bc@kernel.dk> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [10.202.226.46] X-ClientProxiedBy: lhreml720-chm.china.huawei.com (10.201.108.71) To lhreml724-chm.china.huawei.com (10.201.108.75) X-CFilter-Loop: Reflected Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 27/11/2019 14:21, Jens Axboe wrote: > On 11/27/19 6:05 AM, John Garry wrote: >> On 27/11/2019 01:46, Jens Axboe wrote: >>>>> Would be interesting to check the generated code for that, ideally >>>>> we'd >>>>> get rid of the extra load for that case, even if it is in the same >>>>> cacheline. >>>>> >>>> I checked the disassembly and we still have the load instead of the >>>> add. >>>> >>>> This is not surprising, as the compiler would not know for certain that >>>> we point to a field within the same struct. But at least we still >>>> should >>>> point to a close memory. >>>> >>>> Note that the pointer could be dropped, which would remove the load, >>>> but >>>> then we have many if-elses which could be slower, not to mention that >>>> the blk-mq-tag code deals in bitmap pointers anyway. >> >> Hi Jens, >> >>> It might still be worthwhile to do: >>> >>> if (tags->ptr == &tags->__default) >>>     foo(&tags->__default); >>> >>> to make it clear, as that branch will predict easily. >> >> Not sure. So this code does produce the same assembly, as we still need >> to do the tags->ptr load for the comparison. > Hi Jens, > How can it be the same? The approach in the patchset needs to load > *tags->ptr, this one needs tags->ptr. That's the big difference. > In the patch for this thread, we have: @@ -121,10 +121,10 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data) WARN_ON_ONCE(1); return BLK_MQ_TAG_FAIL; } - bt = &tags->breserved_tags; + bt = tags->breserved_tags; tag_offset = 0; } else { - bt = &tags->bitmap_tags; + bt = tags->bitmap_tags; tag_offset = tags->nr_reserved_tags; } So current code gets bt pointer by simply offsetting a certain distance from tags pointer - that is the add I mention. With the change in this patch, we need to load memory at address &tags->bitmap_tags to get bt - this is the load I mention. So for this: if (tags->ptr == &tags->__default) We load &tags->ptr to get the pointer value for comparison vs &tags->__default. There must be something I'm missing... Thanks, John