From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751366AbdJEMA5 (ORCPT ); Thu, 5 Oct 2017 08:00:57 -0400 Received: from mailout1.samsung.com ([203.254.224.24]:44014 "EHLO mailout1.samsung.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751295AbdJEMAy (ORCPT ); Thu, 5 Oct 2017 08:00:54 -0400 X-AuditID: b6c32a36-c21ff70000018158-47-59d61ef24fed From: Bartlomiej Zolnierkiewicz To: "Hunter, Adrian" Cc: Linus Walleij , Ulf Hansson , linux-mmc , linux-block , linux-kernel , Bough Chen , Alex Lemberg , "Nowak, Mateusz" , Yuliy Izrailov , Jaehoon Chung , Dong Aisheng , Das Asutosh , Zhangfei Gao , Sahitya Tummala , Harjani Ritesh , Venu Byravarasu , Shawn Lin , Christoph Hellwig Subject: Re: [PATCH V9 13/15] mmc: block: Add CQE and blk-mq support Date: Thu, 05 Oct 2017 14:00:48 +0200 Message-id: <1591139.5Ihy91Pq6o@amdc3058> User-Agent: KMail/4.13.3 (Linux/3.13.0-96-generic; KDE/4.13.3; x86_64; ; ) In-reply-to: <363DA0ED52042842948283D2FC38E4649BF33EFD@IRSMSX106.ger.corp.intel.com> MIME-version: 1.0 Content-transfer-encoding: 7Bit Content-type: text/plain; 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Ulf Hansson > > ; linux-mmc ; linux- > > block ; linux-kernel > kernel@vger.kernel.org>; Bough Chen ; Alex > > Lemberg ; Nowak, Mateusz > > ; Yuliy Izrailov ; > > Jaehoon Chung ; Dong Aisheng > > ; Das Asutosh ; > > Zhangfei Gao ; Sahitya Tummala > > ; Harjani Ritesh ; > > Venu Byravarasu ; Shawn Lin > chips.com>; Christoph Hellwig > > Subject: Re: [PATCH V9 13/15] mmc: block: Add CQE and blk-mq support > > > > > > Hi, > > > > On Wednesday, October 04, 2017 09:39:45 AM Linus Walleij wrote: > > > On Fri, Sep 22, 2017 at 2:37 PM, Adrian Hunter > > wrote: > > > > > > > Add CQE support to the block driver, including: > > > > - optionally using DCMD for flush requests > > > > - "manually" issuing discard requests > > > > - issuing read / write requests to the CQE > > > > - supporting block-layer timeouts > > > > - handling recovery > > > > - supporting re-tuning > > > > > > > > CQE offers 25% - 50% better random multi-threaded I/O. There is a > > > > slight (e.g. 2%) drop in sequential read speed but no observable > > > > change to sequential write. > > > > > > > > CQE automatically sends the commands to complete requests. However > > > > it only supports reads / writes and so-called "direct commands" > > > > (DCMD). Furthermore DCMD is limited to one command at a time, but > > discards require 3 commands. > > > > That makes issuing discards through CQE very awkward, but some CQE's > > > > don't support DCMD anyway. So for discards, the existing non-CQE > > > > approach is taken, where the mmc core code issues the 3 commands one > > at a time i.e. > > > > mmc_erase(). Where DCMD is used, is for issuing flushes. > > > > > > > > For host controllers without CQE support, blk-mq support is extended > > > > to synchronous reads/writes or, if the host supports > > > > CAP_WAIT_WHILE_BUSY, asynchonous reads/writes. The advantage of > > > > asynchronous reads/writes is that it allows the preparation of the > > > > next request while the current request is in progress. > > > > > > > > Signed-off-by: Adrian Hunter > > > > > > I am trying to wrap my head around this large patch. The size makes it > > > hard but I am doing my best. > > > > I also think that this patch should be split on two patches. The 1st one > > introducing blk-mq and the 2nd one adding CQE support. > > > > [ I don't agree that they make more sense together, on the contrary, > > it is very difficult to properly analyze blk-mq changes on their > > own while there are mixed with CQE related ones. ] > > The CQE and non-CQE code paths are clearly marked. But maybe you The combined patch is > 1 kLOC which is a lot and since the CQE and non-CQE code paths are clearly marked it should be really easy to split them. > are asking what the code would look like if we *never* had to support CQE. > And my point is we *do* have to support CQE. We *do* but not in the same step, it is just normal kernel engineering practice to split patches on logical changes. This is not asking about something extraordinary. > Do you have any questions about the code? I would really hope to see more documentation about blk-mq changes first before starting to review it. The patch description documents CQE changes but not blk-mq ones and the code itself is also lacking in-code documentation describing new functions added for blk-mq. Best regards, -- Bartlomiej Zolnierkiewicz Samsung R&D Institute Poland Samsung Electronics