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=-10.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=unavailable 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 D0A54C433DB for ; Thu, 14 Jan 2021 09:17:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7E0FF2242A for ; Thu, 14 Jan 2021 09:17:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727437AbhANJRP (ORCPT ); Thu, 14 Jan 2021 04:17:15 -0500 Received: from out30-131.freemail.mail.aliyun.com ([115.124.30.131]:51902 "EHLO out30-131.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726478AbhANJRN (ORCPT ); Thu, 14 Jan 2021 04:17:13 -0500 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R141e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04426;MF=jefflexu@linux.alibaba.com;NM=1;PH=DS;RN=4;SR=0;TI=SMTPD_---0ULhhF75_1610615776; Received: from admindeMacBook-Pro-2.local(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0ULhhF75_1610615776) by smtp.aliyun-inc.com(127.0.0.1); Thu, 14 Jan 2021 17:16:17 +0800 Subject: Re: [dm-devel] [PATCH RFC 6/7] block: track cookies of split bios for bio-based device To: Mike Snitzer Cc: linux-block@vger.kernel.org, dm-devel@redhat.com, io-uring@vger.kernel.org References: <20201223112624.78955-1-jefflexu@linux.alibaba.com> <20201223112624.78955-7-jefflexu@linux.alibaba.com> <20210107221825.GF21239@redhat.com> <97ec2025-4937-b476-4f15-446cc304e799@linux.alibaba.com> <20210108172635.GA29915@redhat.com> <16ba3a63-86f5-1acd-c129-767540186689@linux.alibaba.com> <20210112161320.GA13931@redhat.com> From: JeffleXu Message-ID: <56e1f2a2-9300-e3c8-4013-9d371385a082@linux.alibaba.com> Date: Thu, 14 Jan 2021 17:16:16 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <20210112161320.GA13931@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 1/13/21 12:13 AM, Mike Snitzer wrote: > On Tue, Jan 12 2021 at 12:46am -0500, > JeffleXu wrote: > >> >> >> On 1/9/21 1:26 AM, Mike Snitzer wrote: >>> On Thu, Jan 07 2021 at 10:08pm -0500, >>> JeffleXu wrote: >>> >>>> Thanks for reviewing. >>>> >>>> >>>> On 1/8/21 6:18 AM, Mike Snitzer wrote: >>>>> On Wed, Dec 23 2020 at 6:26am -0500, >>>>> Jeffle Xu wrote: >>>>> >>>>>> This is actuaaly the core when supporting iopoll for bio-based device. >>>>>> >>>>>> A list is maintained in the top bio (the original bio submitted to dm >>>>>> device), which is used to maintain all valid cookies of split bios. The >>>>>> IO polling routine will actually iterate this list and poll on >>>>>> corresponding hardware queues of the underlying mq devices. >>>>>> >>>>>> Signed-off-by: Jeffle Xu >>>>> >>>>> Like I said in response to patch 4 in this series: please fold patch 4 >>>>> into this patch and _really_ improve this patch header. >>>>> >>>>> In particular, the (ab)use of bio_inc_remaining() needs be documented in >>>>> this patch header very well. >>>>> >>>>> But its use could easily be why you're seeing a performance hit (coupled >>>>> with the extra spinlock locking and list management used). Just added >>>>> latency and contention across CPUs. >>>> >>>> Indeed bio_inc_remaining() is abused here and the code seems quite hacky >>>> here. >>>> >>>> Actually I'm regarding implementing the split bio tracking mechanism in >>>> a recursive way you had ever suggested. That is, the split bios could be >>>> maintained in an array, which is allocated with 'struct dm_io'. This way >>>> the overhead of spinlock protecting the &root->bi_plist may be omitted >>>> here. Also the lifetime management may be simplified somehow. But the >>>> block core needs to fetch the per-bio private data now, just like what >>>> you had ever suggested before. >>>> >>>> How do you think, Mike? >>> >>> Yes, using per-bio-data is a requirement (we cannot bloat 'struct bio'). >> >> Agreed. Then MD will need some refactor to support IO polling, if >> possible, since just like I mentioned in patch 0 before, MD doesn't >> allocate extra clone bio, and just re-uses the original bio structure. >> >> >>> >>> As for using an array, how would you index the array? >> >> The 'array' here is not an array of 'struct blk_mq_hw_ctx *' maintained >> in struct dm_table as you mentioned. Actually what I mean is to maintain >> an array of struct dm_poll_data (or something like that, e.g. just >> struct blk_mq_hw_ctx *) in per-bio private data. The size of the array >> just equals the number of the target devices. >> >> For example, for the following device stack, >> >>>> >>>> Suppose we have the following device stack hierarchy, that is, dm0 is >>>> stacked on dm1, while dm1 is stacked on nvme0 and nvme1. >>>> >>>> dm0 >>>> dm1 >>>> nvme0 nvme1 >>>> >>>> >>>> Then the bio graph is like: >>>> >>>> >>>> +------------+ >>>> |bio0(to dm0)| >>>> +------------+ >>>> ^ >>>> | orig_bio >>>> +--------------------+ >>>> |struct dm_io A | >>>> +--------------------+ bi_private ---------------------- >>>> |bio3(to dm1) |------------>|bio1(to dm1) | >>>> +--------------------+ +--------------------+ >>>> ^ ^ >>>> | ->orig_bio | ->orig_bio >>>> +--------------------+ +--------------------+ >>>> |struct dm_io | |struct dm_io B | >>>> ---------------------- ---------------------- >>>> |bio2(to nvme0) | |bio4(to nvme1) | >>>> +--------------------+ +--------------------+ >>>> >> >> An array of struct blk_mq_hw_ctx * is maintained in struct dm_io B. >> >> >> struct blk_mq_hw_ctx * hctxs[2]; >> >> The array size is two since dm1 maps to two target devices (i.e. nvme0 >> and nvme1). Then hctxs[0] points to the hw queue of nvme0, while >> hctxs[1] points to the hw queue of nvme1. > > Both nvme0 and nvme1 may have multiple hctxs. Not sure why you're > thinking there is just one per device? > >> >> >> This mechanism supports arbitrary device stacking. Similarly, an array >> of struct blk_mq_hw_ctx * is maintained in struct dm_io A. The array >> size is one since dm0 only maps to one target device (i.e. dm1). In this >> case, hctx[0] points to the struct dm_io of the next level, i.e. struct >> dm_io B. >> >> >> But I'm afraid the implementation of this style may be more complex. > > We are running the risk of talking in circles about this design... Sorry for the inconvenience. I have started working on the next version, but I do want to clarify some design issues first. > > >>>> struct node { >>>> struct blk_mq_hw_ctx *hctx; >>>> blk_qc_t cookie; >>>> }; >>> >>> Needs a better name, think I had 'struct dm_poll_data' >> >> Sure, the name here is just for example. >> >> >>> >>>> Actually currently the tracking objects are all allocated with 'struct >>>> bio', then the lifetime management of the tracking objects is actually >>>> equivalent to lifetime management of bio. Since the returned cookie is >>>> actually a pointer to the bio, the refcount of this bio must be >>>> incremented, since we release a reference to this bio through the >>>> returned cookie, in which case the abuse of the refcount trick seems >>>> unavoidable? Unless we allocate the tracking object individually, then >>>> the returned cookie is actually pointing to the tracking object, and the >>>> refcount is individually maintained for the tracking object. >>> >>> The refcounting and lifetime of the per-bio-data should all work as is. >>> Would hope you can avoid extra bio_inc_remaining().. that infratsructure >>> is way too tightly coupled to bio_chain()'ing, etc. >>> >>> The challenge you have is the array that would point at these various >>> per-bio-data needs to be rooted somewhere (you put it in the topmost >>> original bio with the current patchset). But why not manage that as >>> part of 'struct mapped_device'? It'd need proper management at DM table >>> reload boundaries and such but it seems like the most logical place to >>> put the array. But again, this array needs to be dynamic.. so thinking >>> further, maybe a better model would be to have a fixed array in 'struct >>> dm_table' for each hctx associated with a blk_mq _data_ device directly >>> used/managed by that dm_table? >> Confusion also stated in the following comment. How 'struct dm_poll_data' could be involved with 'struct dm_table' or 'struct mapped_device'. In the current patchset, every bio need to maintain one list to track all its 'struct dm_poll_data' structures. Then how to maintain this per-bio information in one single 'struct dm_table' or 'struct mapped_device'? >> It seems that you are referring 'array' here as an array of 'struct >> blk_mq_hw_ctx *'? Such as >> >> struct dm_table { >> ... >> struct blk_mq_hw_ctx *hctxs[]; >> }; >> >> Certainly with this we can replace the original 'struct blk_mq_hw_ctx *' >> pointer in 'struct dm_poll_data' with the index into this array, such as >> >> struct dm_poll_data { >> int hctx_index; /* index into dm_table->hctxs[] */ >> blk_qc_t cookie; >> }; > > You seized on my mentioning blk-mq's array of hctx too literally. I was > illustrating that blk-mq's cookie is converted to an index into that > array. > > But for this DM bio-polling application we'd need to map the blk-mq > returned cookie to a request_queue. Hence the original 2 members of > dm_poll_data needing to be 'struct request_queue *' and blk_qc_t. > >> But I'm doubted if this makes much sense. The core difficulty here is >> maintaining a list (or dynamic sized array) to track all split bios. >> With the array of 'struct blk_mq_hw_ctx *' maintained in struct >> dm_table, we still need some **per-bio** structure (e.g., &bio->bi_plist >> in current patch set) to track these split bios. > > One primary goal of all of this design is to achieve bio-polling cleanly > (without extra locking, without block core data structure bloat, etc). > I know you share that goal. But we need to nail down the core data > structures and what needs tracking at scale and then associate them with > DM's associated objects with consideration for object lifetime. > > My suggestion was to anchor your core data structures (e.g. 'struct > dm_poll_data' array, etc) to 'struct dm_table'. I suggested that > because the dm_table is what dm_get_device()s each underlying _data_ > device (a subset of all devices in a dm_table, as iterated through > .iterate_devices). But a DM 'struct mapped_device' has 2 potential > dm_tables, active and inactive slots, that would imply some complexity > in handing off any outstanding bio's associated 'struct dm_poll_data' > array on DM table reload. 1) If 'struct dm_poll_data' resides in per-bio-data, then how do you **link** or **associate** all the 'struct dm_poll_data' structures from one original bio? Do we link them by the internal relationship between bio/dm_io/dm_target_io, or some other auxiliary data structure? 2) I get confused how 'struct dm_poll_data' could be involved with 'struct dm_table'. Is there an array of 'struct dm_poll_data' or 'struct dm_poll_data *' maintained in 'struct dm_table'? If this is the case, then the size of the array may be incredible large, or expanded/shrank frequently, since one dm_table could correspond to millions bios. > > Anyway, you seem to be gravitating to a more simplistic approach of a > single array of 'struct dm_poll_data' for each DM device (regardless of > how arbitrarily deep that DM device stack is, the topmost DM device > would accumulate the list of 'struct dm_poll_data'?). I'm open to this. At least you don't need to care the lifetime of other disparate 'struct dm_poll_data's, if all 'struct dm_poll_data's are accumulated in one (e.g., the topmost) place. > > I'm now questioning the need for any high-level data structure to track > all N of the 'struct dm_poll_data' that may result from a given bio (as > it is split to multiple blk-mq hctxs across multiple blk-mq devices). > Each 'struct dm_poll_data', that will be returned to block core and > stored in struct kiocb's ki_cookie, would have an object lifetime that > matches the original DM bio clone's per-bio-data that the 'struct > dm_poll_data' was part of; then we just need to cast that ki_cookie's > blk_qc_t as 'struct dm_poll_data' and call blk_poll(). > > The hardest part is to ensure that all the disparate 'struct > dm_poll_data' (and associated clone bios) aren't free'd until the > _original_ bio completes. That would create quite some back-pressure > with more potential to exhaust system resources -- because then the > cataylst for dropping reference counts on these clone bios would then > need to be tied to the blk_bio_poll() interface... which feels "wrong" > (e.g. it ushers in the (ab)use of bio_inc_remaining you had in your most > recent patchset). > > All said, maybe post a v2 that takes the incremental steps of: > 1) using DM per-bio-data for 'struct dm_poll_data' > 2) simplify blk_bio_poll() to call into DM to translate provided > blk_qc_t (from struct kiocb's ki_cookie) to request_queue and > blk_qc_t. > - this eliminates any need for extra list processing > 3) keep your (ab)use of bio_inc_remaining() to allow for exploring this -- Thanks, Jeffle