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=-2.3 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 A4EF1C2D0CD for ; Tue, 17 Dec 2019 15:05:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 84C972067C for ; Tue, 17 Dec 2019 15:05:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728275AbfLQPFy (ORCPT ); Tue, 17 Dec 2019 10:05:54 -0500 Received: from mx2.suse.de ([195.135.220.15]:33896 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727736AbfLQPFw (ORCPT ); Tue, 17 Dec 2019 10:05:52 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 64E1EAD69; Tue, 17 Dec 2019 15:05:50 +0000 (UTC) Received: by ds.suse.cz (Postfix, from userid 10065) id D0476DA81D; Tue, 17 Dec 2019 16:05:48 +0100 (CET) Date: Tue, 17 Dec 2019 16:05:48 +0100 From: David Sterba To: Dennis Zhou Cc: David Sterba , Chris Mason , Josef Bacik , kernel-team@fb.com, linux-btrfs@vger.kernel.org Subject: Re: [PATCH v2 2/2] btrfs: fix compressed write bio attribution Message-ID: <20191217150548.GF3929@suse.cz> Reply-To: dsterba@suse.cz Mail-Followup-To: dsterba@suse.cz, Dennis Zhou , David Sterba , Chris Mason , Josef Bacik , kernel-team@fb.com, linux-btrfs@vger.kernel.org References: <20191212181934.GA33645@dennisz-mbp.dhcp.thefacebook.com> <20191213122401.GV3929@suse.cz> <20191213222149.GA46346@dennisz-mbp.dhcp.thefacebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191213222149.GA46346@dennisz-mbp.dhcp.thefacebook.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Fri, Dec 13, 2019 at 02:21:49PM -0800, Dennis Zhou wrote: > On Fri, Dec 13, 2019 at 01:24:01PM +0100, David Sterba wrote: > > On Thu, Dec 12, 2019 at 10:19:34AM -0800, Dennis Zhou wrote: > > > From a0569aebde08e31e994c92d0b70befb84f7f5563 Mon Sep 17 00:00:00 2001 > > > From: Dennis Zhou > > > Date: Wed, 11 Dec 2019 15:20:15 -0800 > > > > > > Bio attribution is handled at bio_set_dev() as once we have a device, we > > > have a corresponding request_queue and then can derive the current css. > > > In special cases, we want to attribute to bio to someone else. This can > > > be done by calling bio_associate_blkg_from_css() or > > > kthread_associate_blkcg() depending on the scenario. Btrfs does this for > > > compressed writeback as they are handled by kworkers, so the latter can > > > be done here. > > > > > > Commit 1a41802701ec ("btrfs: drop bio_set_dev where not needed") removes > > > early bio_set_dev() calls prior to submit_stripe_bio(). This breaks the > > > above assumption that we'll have a request_queue when we are doing > > > association. To fix this, switch to using kthread_associate_blkcg(). > > > > Can be kthread_associate_blkcg used also for submit_extent_page that > > calls bio_associate_blkg_from_css indirectly when initializing wbc? > > > > 2996 bio_set_dev(bio, bdev); > > 2997 wbc_init_bio(wbc, bio); > > 2998 wbc_account_cgroup_owner(wbc, page, page_size); > > > > wbc_init_bio: > > > > if (wbc) > > bio_associate_blkg_from_css(); > > Correct me if I'm wrong, but I don't think submit_extent_page() is only > called from kthread contexts. So, we wouldn't be able to rely on > kthread_associate_blkcg(). Yeah, the kthread is not guaranteed here. > I can think about how to make wbc better for association in general, but > it's a percpu decrement and increment so it shouldn't really be much in > overhead. Performance is not my concern here, the addition of bios and blkcg association is new and there were some integration bugs where I independently removed early bdev association while the blkg relied on that. I'm looking for ways to make it less error prone and the kthread association looks exactly like that so I was curious if it's possible to use it everywhere. If not, the bdev needs to be found from other available data.