From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757230Ab2BBUzi (ORCPT ); Thu, 2 Feb 2012 15:55:38 -0500 Received: from mx1.redhat.com ([209.132.183.28]:57756 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753525Ab2BBUzh (ORCPT ); Thu, 2 Feb 2012 15:55:37 -0500 Date: Thu, 2 Feb 2012 15:55:32 -0500 From: Vivek Goyal To: Tejun Heo Cc: axboe@kernel.dk, ctalbott@google.com, rni@google.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 01/11] blkcg: let blkio_group point to blkio_cgroup directly Message-ID: <20120202205532.GD1723@redhat.com> References: <1328131156-13290-1-git-send-email-tj@kernel.org> <1328131156-13290-2-git-send-email-tj@kernel.org> <20120202200318.GB1723@redhat.com> <20120202203352.GG19837@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120202203352.GG19837@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 02, 2012 at 12:33:52PM -0800, Tejun Heo wrote: > Hello, > > On Thu, Feb 02, 2012 at 03:03:18PM -0500, Vivek Goyal wrote: > > > +static void blkiocg_destroy(struct cgroup_subsys *subsys, struct cgroup *cgroup) > > > +{ > > > + struct blkio_cgroup *blkcg = cgroup_to_blkio_cgroup(cgroup); > > > + > > > if (blkcg != &blkio_root_cgroup) > > > kfree(blkcg); > > > > What makes sure that all the blkg are gone and they have dropped their > > reference to blkcg? IIUC, pre-destroy will just make sure to decouple > > blkg from request queue as well as blkcg list and also drop joint cgroup > > and request queue reference. > > > > But there could well be some IO queued in the group which might have > > its own reference and will be dropped later when IO completes. So at > > the time of blkiocg_destroy() it is not guranteed that there are > > no reference holders to blkcg? > > Yeah, and that would wait till all css refs held by blkgs are gone, > right? Ok, I missed that. So ->destroy() is not called till all the css refs are gone. So directory removal will wait for all the pending IO in the group to finish. I have some minor concerns here. A low prio group might not be able to dispatch IO for quite some amount of time. Especially with CFQ, in presence of sync IO, async IO can be starved for a very long time. So if some task dumped bunch of low prio IO and exited and now we are removing the cgroup, this task might have to wait for significant amount of time. (I am worried about "hung task waited for 120 seconds kind of messages"). Also it is little unintutive to the user that why rmdir should be delayed. Anyway, this is not a very strong concern. Just something to keep in mind. Thanks Vivek