From patchwork Tue Jun 16 22:48:32 2015 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Tejun Heo X-Patchwork-Id: 6620951 Return-Path: X-Original-To: patchwork-linux-fsdevel@patchwork.kernel.org Delivered-To: patchwork-parsemail@patchwork1.web.kernel.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.136]) by patchwork1.web.kernel.org (Postfix) with ESMTP id 7D2229F326 for ; Tue, 16 Jun 2015 22:50:39 +0000 (UTC) Received: from mail.kernel.org (localhost [127.0.0.1]) by mail.kernel.org (Postfix) with ESMTP id 8E3942080F for ; Tue, 16 Jun 2015 22:50:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7C157207E7 for ; Tue, 16 Jun 2015 22:50:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752014AbbFPWtO (ORCPT ); Tue, 16 Jun 2015 18:49:14 -0400 Received: from mail-yk0-f173.google.com ([209.85.160.173]:33886 "EHLO mail-yk0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755524AbbFPWtD (ORCPT ); Tue, 16 Jun 2015 18:49:03 -0400 Received: by ykfl8 with SMTP id l8so26088099ykf.1; Tue, 16 Jun 2015 15:49:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=sender:from:to:cc:subject:date:message-id:in-reply-to:references; bh=eGQsSnZdhNJkHyIj3BcBLCxbKr+CI7YSLQFojeBTOSY=; b=MzioWz1USBjXEUn8ob2sbyeULhWtmXL9iHjpOIYNB+frjNmcjkM22JrmibDKxZMPi8 IvLAC2BZlJEaM2dTNBb2/smc39kInsREkKR8HA21SD1mkdNyRM6fuDMXCz71645blSE5 UEFehGufKfXPkiLTir+JLYmIq3MiEDhfiIkGwTDH4I+3AO0wIWrEaWHRyXXbq02vAQx8 4376s97nE6p279Po6Ry+sPcT0bkHjRgtJ2DmayErwop3G+ZWc2HuXkKmKLblrDF4uis7 /eqk8ZgOePuMzHnvFrAmIuNvd4aU0sKoEQacY4kOX2FJjf2IuA0end+F4B9hvzVW67Xp CNPg== X-Received: by 10.13.219.78 with SMTP id d75mr1681497ywe.170.1434494942769; Tue, 16 Jun 2015 15:49:02 -0700 (PDT) Received: from mtj.duckdns.org.DHCP.TheFacebook.com ([199.201.65.130]) by mx.google.com with ESMTPSA id k41sm979569yho.29.2015.06.16.15.49.00 (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 Jun 2015 15:49:02 -0700 (PDT) From: Tejun Heo To: axboe@kernel.dk Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, lizefan@huawei.com, cgroups@vger.kernel.org, hannes@cmpxchg.org, kernel-team@fb.com, Tejun Heo , Vivek Goyal Subject: [PATCH 3/3] writeback, blkio: add documentation for cgroup writeback support Date: Tue, 16 Jun 2015 18:48:32 -0400 Message-Id: <1434494912-31043-4-git-send-email-tj@kernel.org> X-Mailer: git-send-email 2.4.3 In-Reply-To: <1434494912-31043-1-git-send-email-tj@kernel.org> References: <1434494912-31043-1-git-send-email-tj@kernel.org> Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org X-Spam-Status: No, score=-7.4 required=5.0 tests=BAYES_00,DKIM_SIGNED, RCVD_IN_DNSWL_HI,RP_MATCHES_RCVD,T_DKIM_INVALID,UNPARSEABLE_RELAY autolearn=unavailable version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on mail.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Update Documentation/cgroups/blkio-controller.txt to reflect the recently added cgroup writeback support. Signed-off-by: Tejun Heo Cc: Li Zefan Cc: Vivek Goyal Cc: cgroups@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org --- Documentation/cgroups/blkio-controller.txt | 83 ++++++++++++++++++++++++++++-- 1 file changed, 78 insertions(+), 5 deletions(-) diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt index cd556b9..68b6a6a 100644 --- a/Documentation/cgroups/blkio-controller.txt +++ b/Documentation/cgroups/blkio-controller.txt @@ -387,8 +387,81 @@ groups and put applications in that group which are not driving enough IO to keep disk busy. In that case set group_idle=0, and CFQ will not idle on individual groups and throughput should improve. -What works -========== -- Currently only sync IO queues are support. All the buffered writes are - still system wide and not per group. Hence we will not see service - differentiation between buffered writes between groups. +Writeback +========= + +Page cache is dirtied through buffered writes and shared mmaps and +written asynchronously to the backing filesystem by the writeback +mechanism. Writeback sits between the memory and IO domains and +regulates the proportion of dirty memory by balancing dirtying and +write IOs. + +On traditional cgroup hierarchies, relationships between different +controllers cannot be established making it impossible for writeback +to operate accounting for cgroup resource restrictions and all +writeback IOs are attributed to the root cgroup. + +If both the blkio and memory controllers are used on the v2 hierarchy +and the filesystem supports cgroup writeback, writeback operations +correctly follow the resource restrictions imposed by both memory and +blkio controllers. + +Writeback examines both system-wide and per-cgroup dirty memory status +and enforces the more restrictive of the two. Also, writeback control +parameters which are absolute values - vm.dirty_bytes and +vm.dirty_background_bytes - are distributed across cgroups according +to their current writeback bandwidth. + +There's a peculiarity stemming from the discrepancy in ownership +granularity between memory controller and writeback. While memory +controller tracks ownership per page, writeback operates on inode +basis. cgroup writeback bridges the gap by tracking ownership by +inode but migrating ownership if too many foreign pages, pages which +don't match the current inode ownership, have been encountered while +writing back the inode. + +This is a conscious design choice as writeback operations are +inherently tied to inodes making strictly following page ownership +complicated and inefficient. The only use case which suffers from +this compromise is multiple cgroups concurrently dirtying disjoint +regions of the same inode, which is an unlikely use case and decided +to be unsupported. Note that as memory controller assigns page +ownership on the first use and doesn't update it until the page is +released, even if cgroup writeback strictly follows page ownership, +multiple cgroups dirtying overlapping areas wouldn't work as expected. +In general, write-sharing an inode across multiple cgroups is not well +supported. + +Filesystem support for cgroup writeback +--------------------------------------- + +A filesystem can make writeback IOs cgroup-aware by updating +address_space_operations->writepage[s]() to annotate bio's using the +following two functions. + +* wbc_init_bio(@wbc, @bio) + + Should be called for each bio carrying writeback data and associates + the bio with the inode's owner cgroup. Can be called anytime + between bio allocation and submission. + +* wbc_account_io(@wbc, @page, @bytes) + + Should be called for each data segment being written out. While + this function doesn't care exactly when it's called during the + writeback session, it's the easiest and most natural to call it as + data segments are added to a bio. + +With writeback bio's annotated, cgroup support can be enabled per +super_block by setting MS_CGROUPWB in ->s_flags. This allows for +selective disabling of cgroup writeback support which is helpful when +certain filesystem features, e.g. journaled data mode, are +incompatible. + +wbc_init_bio() binds the specified bio to its cgroup. Depending on +the configuration, the bio may be executed at a lower priority and if +the writeback session is holding shared resources, e.g. a journal +entry, may lead to priority inversion. There is no one easy solution +for the problem. Filesystems can try to work around specific problem +cases by skipping wbc_init_bio() or using bio_associate_blkcg() +directly.