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=-5.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 5CAEFC2D0A3 for ; Fri, 6 Nov 2020 18:46:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E5F0B2067B for ; Fri, 6 Nov 2020 18:46:02 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Bcd7Yb6w" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727800AbgKFSqC (ORCPT ); Fri, 6 Nov 2020 13:46:02 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:42762 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727069AbgKFSqC (ORCPT ); Fri, 6 Nov 2020 13:46:02 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1604688359; 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=KYXMgpgXis5HDOFrwXjTVkW33lbZU10WK2LjptI7g1c=; b=Bcd7Yb6wjkkZOHCp+kHveyiWK8WUndSqfMOiNXlRS12tSwHVgbl+56POZU2w6rUNVahBI/ qmdFUWg794iUFun5XexoTV69HXc5eR+B96u97Yx7p5vJlXCMcKKPNimvUOE3sIB6+3qd3Z r8nw+OiIxXVskp/10z039UwBOX7gVkM= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-399-sfCphrnjMTe-EAyIPf8bSA-1; Fri, 06 Nov 2020 13:45:57 -0500 X-MC-Unique: sfCphrnjMTe-EAyIPf8bSA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id B4F8B6D249; Fri, 6 Nov 2020 18:45:55 +0000 (UTC) Received: from localhost (unknown [10.18.25.174]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1D5E876645; Fri, 6 Nov 2020 18:45:52 +0000 (UTC) Date: Fri, 6 Nov 2020 12:45:26 -0500 From: Mike Snitzer To: JeffleXu Cc: axboe@kernel.dk, xiaoguang.wang@linux.alibaba.com, linux-block@vger.kernel.org, joseph.qi@linux.alibaba.com, dm-devel@redhat.com, haoxu@linux.alibaba.com, io-uring@vger.kernel.org Subject: Re: [RFC 0/3] Add support of iopoll for dm device Message-ID: <20201106174526.GA13292@redhat.com> References: <20201020065420.124885-1-jefflexu@linux.alibaba.com> <20201021203906.GA10896@redhat.com> <20201026185334.GA8463@redhat.com> <33c32cd1-5116-9a42-7fe2-b2a383f1c7a0@linux.alibaba.com> <20201102152822.GA20466@redhat.com> <20201104150847.GB32761@redhat.com> <2c5dab21-8125-fcdf-078e-00a158c57f43@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <2c5dab21-8125-fcdf-078e-00a158c57f43@linux.alibaba.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Thu, Nov 05 2020 at 9:51pm -0500, JeffleXu wrote: > > On 11/4/20 11:08 PM, Mike Snitzer wrote: > >>I'm doubted if this should be implemented in block layer like: > >> > >>``` > >> > >>struct bio { > >> > >>     ... > >> > >>     struct list_head  cookies; > >> > >>}; > >> > >>``` > >> > >>After all it's only used by bio-based queue, or more specifically > >>only dm device currently. > >I do think this line of work really should be handled in block core > >because I cannot see any reason why MD or bcache or whatever bio-based > >device wouldn't want the ability to better support io_uring (with IO > >poll). > > > >>Another design I can come up with is to maintain a global data > >>structure for the very beginning > >>original bio. Currently the blocking point is that now one original > >>bio to the dm device (@bio of dm_submit()) can correspond to multiple > >>dm_io and thus we have nowhere to place the @cookies list. > >Yes, and that will always be the case. We need the design to handle an > >arbitrary sprawl of splitting from a given bio. The graph of bios > >resulting from that fan-out needs to be walked at various levels -- be > >it the top-level original bio's submit_bio() returned cookie or some > >intermediate point in the chain of bios. > > > >The problem is the lifetime of the data structure created for a given > >split bio versus layering boundaries (that come from block core's > >simplistic recursion via bio using submit_bio). > > > >>Now we have to maintain one data structure for every original bio, > >>something like > >> > >>``` > >> > >>struct dm_poll_instance { > >> > >>     ... > >> > >>     struct list_head cookies; > >> > >>}; > >> > >>``` > >I do think we need a hybrid where at the point of recursion we're able > >to make the associated data structure available across the recursion > >boundary so that modeling the association in a chain of split bios is > >possible. (e.g. struct dm_poll_data or dm_poll_instance as you named it, > >_but_ that struct definition would live in block core, but would be part > >of per-bio-data; so 'struct blk_poll_data' is more logical name when > >elevated to block core). > > > >It _might_ be worthwhile to see if a new BIO_ flag could be added to > >allow augmenting the bio_split + bio_chain pattern to also track this > >additional case of carrying additional data per-bio while creating > >bio-chains. I may not be clear yet, said differently: augmenting > >bio_chain to not only chain bios, but to _also_ thread/chain together > >per-bio-data that lives within those chained bios. SO you have the > >chain of bios _and_ the chain of potentially opaque void * that happens > >to point to a list head for a list of 'struct blk_poll_data'. > > > >Does that make sense? > > > I'm doubted if it really makes sense to maintain a *complete* bio > chain across the recursive > > call boundary. > > > Considering the following device stack: > > ``` > >                                   dm0 > >         dm1                   dm2           dm3 > > nvme0  nvme1          ....               .... > > ``` > > > We have the following bio graph (please let me know if it's wrong or > the image can't display) > > > For example, for dm1 there are three bios containing valid cookie in > the end, that is > > bio 9/10/11. We only need to insert the corresponding blk_poll_data > (containing > > request_queue, cookie, etc) of these three bios into the very > beginning original > > bio (that is bio0). Of course we can track all the sub-bios down > through the device > > stack, layer by layer, e.g., > > - get bio 1/2/3 from bio 0 > > - get bio 4 from bio 3 > > - finally get bio 9 from bio 4 > > But I'm doubted if it's overkill to just implement the iopoll. > > > Another issue still unclear is that, if we should implement the > iopoll in a recursive way. > > In a recursive way, to poll dm 0, we should poll all its > sub-devices, that is, bio 4/5/7/8. > > Oppositely we can insert only the bottom bio (bio 9/10/11 which have > valid cookie) at > > the very beginning (at submit_bio() phase), and to poll dm 0, we > only need to poll bio 9/10/11. I feel we need the ability to walk the entire graph and call down to next level. The lowest level would be what you call a "valid cookie" that blk-mq returned. But the preceding cookies need to be made valid and used when walking the graph (from highest to lowest) and calling down to the underlying layers. > > > To implement this non-recursive design, we can add a field in struct bio > > ``` > > struct bio { > >     ... > >     struct bio *orig; > > } > > ``` > > @orig points to the original bio inputted into submit_bio(), that is, bio 0. > > > @orig field is transmitted through bio splitting. > > ``` > > bio_split() > >     split->orig = bio->orig ? : bio > > > dm.c: __clone_and_map_data_bio > >     clone->orig = bio->orig ? : bio > > ``` > > > Finally bio 9/10/11 can be inserted into bio 0. > > ``` > > blk-mq.c: blk_mq_submit_bio > >     if (bio->orig) > >         init blk_poll_data and insert it into bio->orig's @cookies list > > ``` If you feel that is doable: certainly give it a shot. But it is not clear to me how you intend to translate from cookie passed in to ->blk_poll hook (returned from submit_bio) to the saved off bio->orig. What is your cookie strategy in this non-recursive implementation? What will you be returning? Where will you be storing it? Mike