From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754631AbcDVSOH (ORCPT ); Fri, 22 Apr 2016 14:14:07 -0400 Received: from mail-yw0-f194.google.com ([209.85.161.194]:36444 "EHLO mail-yw0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753284AbcDVSOE (ORCPT ); Fri, 22 Apr 2016 14:14:04 -0400 Date: Fri, 22 Apr 2016 14:13:21 -0400 From: Tejun Heo To: Paolo Cc: Jens Axboe , Fabio Checconi , Arianna Avanzini , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, ulf.hansson@linaro.org, linus.walleij@linaro.org, broonie@kernel.org Subject: Re: [PATCH RFC 10/22] block, bfq: add full hierarchical scheduling and cgroups support Message-ID: <20160422181321.GV7822@mtj.duckdns.org> References: <1454364778-25179-1-git-send-email-paolo.valente@linaro.org> <1454364778-25179-11-git-send-email-paolo.valente@linaro.org> <20160211222824.GD3741@mtj.duckdns.org> <57174CA7.5000706@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <57174CA7.5000706@linaro.org> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Paolo. On Wed, Apr 20, 2016 at 11:32:23AM +0200, Paolo wrote: > This malfunction seems related to a blkcg behavior that I did not > expect: the sequential writer changes group continuously. It moves > from the root group to its correct group, and back. Here is the > output of > > egrep 'insert_request|changed cgroup' trace > > over a trace taken with the original version of cfq (seq_write is of > course the group of the writer): ... > For reasons that I don't yet know, group changes are much more > frequent with bfq, which ultimately causes bfq to fail to isolate the > writer from the reader. > > While I go on trying to understand why, could you please tell me > whether this fluctuation is normal, and/or point me to documentation from > which I can better understand this behavior, without bothering you > further? So, a kworker would jump through different workqueues and issue IOs for different writeback domains and the context can't be tied to the issuing task. The cgroup membership should be determined directly from the bio. cfq uses per-cgroup async queue. I'm not sure how this would map to bfq tho. Thanks. -- tejun