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 BA6B4C433F5 for ; Wed, 30 Mar 2022 19:23:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1350620AbiC3TZG (ORCPT ); Wed, 30 Mar 2022 15:25:06 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52466 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S241000AbiC3TZE (ORCPT ); Wed, 30 Mar 2022 15:25:04 -0400 Received: from mail-pf1-f171.google.com (mail-pf1-f171.google.com [209.85.210.171]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5D021237 for ; Wed, 30 Mar 2022 12:23:16 -0700 (PDT) Received: by mail-pf1-f171.google.com with SMTP id x31so13123768pfh.9 for ; Wed, 30 Mar 2022 12:23:16 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=ZHtyaTDCFwT16k5cdDfPF/P2EA5gPFqUR2PCvm4ECgI=; b=YQfob1wgJ8QhpYSvS1CfnB2Df1MJFbEXTXCNza5VUvu8+wThcuvOvK08J/zTZ4Qbdm 1GxBpCLPfS2ilJhgRSeOPcSLxVsNQkIttpUi8oqmd2oUbLxFeIvZxUpMrXZCWE3v/PCo sUzMJPWZK6HKPXpgBTsIJGH4ejQoATg7infOmEj8NG9uxgXORVkwCxBRi67/xp1JyfHL ZmdChelr/5IM3NTkn+5Nw5vOf5Z1g31imRSUT4HqISREpyLAqqz0JRfYA89x66y8mKxc mlvlcwYq6cLW51vYS+ekfpOmIhFppu3Mg5LZU8UUsZ9tZyaZ1MobS8fFifGBCsp3xcAD 73SA== X-Gm-Message-State: AOAM533ytZfYvrRc4tV4AL7ljBEiT2nZ3m524yBHhc2TuWLcYUVLWs2F TIqi+rcREOyNfxn9oNnUeAd0WkrcyMM= X-Google-Smtp-Source: ABdhPJzwW5Uj3tPdbpOaNG5QzaofCKC1qF7aDIo6ArraeWowHZ4NFFbLL3rxw31k/agc3F4ivB+E5w== X-Received: by 2002:a05:6a00:ad0:b0:4f7:a357:6899 with SMTP id c16-20020a056a000ad000b004f7a3576899mr34729945pfl.80.1648668195778; Wed, 30 Mar 2022 12:23:15 -0700 (PDT) Received: from fedora (136-24-99-118.cab.webpass.net. [136.24.99.118]) by smtp.gmail.com with ESMTPSA id h6-20020a056a00218600b004f65315bb37sm26083117pfi.13.2022.03.30.12.23.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 30 Mar 2022 12:23:15 -0700 (PDT) Date: Wed, 30 Mar 2022 08:28:28 -0400 From: Dennis Zhou To: Mike Snitzer Cc: tj@kernel.org, axboe@kernel.dk, linux-block@vger.kernel.org, dm-devel@redhat.com Subject: Re: can we reduce bio_set_dev overhead due to bio_associate_blkg? Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org Hi Mike, On Wed, Mar 30, 2022 at 12:52:58PM -0400, Mike Snitzer wrote: > Hey Tejun and Dennis, > > I recently found that due to bio_set_dev()'s call to > bio_associate_blkg(), bio_set_dev() needs much more cpu than ideal; > especially when doing 4K IOs via io_uring's HIPRI bio-polling. > > I'm very naive about blk-cgroups.. so I'm hopeful you or others can > help me cut through this to understand what the ideal outcome should > be for DM's bio clone + remap heavy use-case as it relates to > bio_associate_blkg. > > If I hack dm-linear with a local __bio_set_dev that simply removes > the call to bio_associate_blkg() my IOPS go from ~980K to 995K. > > Looking at what is happening a bit, relative to this DM bio cloning > usecase, it seems __bio_clone() calls bio_clone_blkg_association() to > clone the blkg from DM device, then dm-linear.c:linear_map's call > to bio_set_dev() will cause bio_associate_blkg(bio) to reuse the css > but then it triggers an update because the bdev is being remapped in > the bio (due to linear_map sending the IO to the real underlying > device). End result _seems_ like collective wasteful effort to get the > blk-cgroup resources setup properly in the face of a simple remap. > > Seems the current DM pattern is causing repeat blkg work for _every_ > remapped bio? Do you see a way to speed up repeat calls to > bio_associate_blkg()? > I must admit I wrote this with limited knowledge of bio cloning at the time. I can fill in the thought process here. The idea was every bio should have a blkg associated with it for io accounting and things like blk-iolatency and blk-iocost. The device abstraction I believe means we can set limits here as well on submission rate to the md device. I think cloning is a special case that I might have gotten wrong. If there is a bio_set_dev() call after each clone(), then the bio_clone_blkg_association() is excess work. We'd need to audit how bio_alloc_clone() is being used to be safe. Alternatively, we could opt for a bio_alloc_clone_noblkg(), but that's a little bit uglier. 1. bio_set_dev() above md <- needed so we can do throttling on the md. 2. bio_alloc_clone() <- doesn't need to clone the blkg() info. 3. bio_set_dev() in md <- sets the right underlying device association. Thanks, Dennis > Test kernel is my latest dm-5.19 branch (though latest Linus 5.18-rc0 > kernel should be fine too): > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/log/?h=dm-5.19 > > I'm using dm-linear ontop on a 16G blk-mq null_blk device: > > modprobe null_blk queue_mode=2 poll_queues=2 bs=4096 gb=16 > SIZE=`blockdev --getsz /dev/nullb0` > echo "0 $SIZE linear /dev/nullb0 0" | dmsetup create linear > > And running the workload with fio using this wrapper script: > io_uring.sh 20 1 /dev/mapper/linear 4096 > > #!/bin/bash > > RTIME=$1 > JOBS=$2 > DEV=$3 > BS=$4 > > QD=64 > BATCH=16 > HI=1 > > fio --bs=$BS --ioengine=io_uring --fixedbufs --registerfiles --hipri=$HI \ > --iodepth=$QD \ > --iodepth_batch_submit=$BATCH \ > --iodepth_batch_complete_min=$BATCH \ > --filename=$DEV \ > --direct=1 --runtime=$RTIME --numjobs=$JOBS --rw=randread \ > --name=test --group_reporting