From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758456AbZFZUQi (ORCPT ); Fri, 26 Jun 2009 16:16:38 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753561AbZFZUQa (ORCPT ); Fri, 26 Jun 2009 16:16:30 -0400 Received: from smtp-out.google.com ([216.239.33.17]:56538 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753548AbZFZUQ3 convert rfc822-to-8bit (ORCPT ); Fri, 26 Jun 2009 16:16:29 -0400 DomainKey-Signature: a=rsa-sha1; s=beta; d=google.com; c=nofws; q=dns; h=mime-version:in-reply-to:references:date:message-id:subject:from:to: cc:content-type:content-transfer-encoding:x-system-of-record; b=uWIWGgn3SM7UPwyWG8qgVnYJRyvaHcVFFZEzeEVR5efTwJ40Ylha05npdIgfCEAJr 8p2v1fkmtklksg5t3QlrQ== MIME-Version: 1.0 In-Reply-To: <20090626141020.849a081e.kamezawa.hiroyu@jp.fujitsu.com> References: <20090623160720.36230fa2.kamezawa.hiroyu@jp.fujitsu.com> <20090626141020.849a081e.kamezawa.hiroyu@jp.fujitsu.com> Date: Fri, 26 Jun 2009 13:16:23 -0700 Message-ID: <6599ad830906261316x52d6c115t720b87ba16b3617@mail.gmail.com> Subject: Re: [PATCH] memcg: cgroup fix rmdir hang From: Paul Menage To: KAMEZAWA Hiroyuki Cc: "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , "nishimura@mxp.nes.nec.co.jp" , "balbir@linux.vnet.ibm.com" , "lizf@cn.fujitsu.com" , "akpm@linux-foundation.org" Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Kamezawa, Sorry that I didn't get a chance to look at these patches before now. On Thu, Jun 25, 2009 at 10:10 PM, KAMEZAWA Hiroyuki wrote: > I hope this will be a final bullet.. > I myself think this one is enough simple and good. > I'm sorry that we need test again. > > == > From: KAMEZAWA Hiroyuki > > After commit: cgroup: fix frequent -EBUSY at rmdir >              ec64f51545fffbc4cb968f0cea56341a4b07e85a > cgroup's rmdir (especially against memcg) doesn't return -EBUSY > by temporal ref counts. That commit expects all refs after pre_destroy() > is temporary but...it wasn't. Then, rmdir can wait permanently. > This patch tries to fix that and change followings. > >  - set CGRP_WAIT_ON_RMDIR flag before pre_destroy(). >  - clear CGRP_WAIT_ON_RMDIR flag when the subsys finds racy case. >   if there are sleeping ones, wakes them up. >  - rmdir() sleeps only when CGRP_WAIT_ON_RMDIR flag is set. > > Changelog v2->v3: >  - removed retry_rmdir() callback. >  - make use of CGRP_WAIT_ON_RMDIR flag more. > > Reported-by: Daisuke Nishimura > Signed-off-by: KAMEZAWA Hiroyuki > --- >  include/linux/cgroup.h |   13 +++++++++++++ >  kernel/cgroup.c        |   38 ++++++++++++++++++++++---------------- >  mm/memcontrol.c        |   25 +++++++++++++++++++++++-- >  3 files changed, 58 insertions(+), 18 deletions(-) > > Index: mmotm-2.6.31-Jun25/include/linux/cgroup.h > =================================================================== > --- mmotm-2.6.31-Jun25.orig/include/linux/cgroup.h > +++ mmotm-2.6.31-Jun25/include/linux/cgroup.h > @@ -366,6 +366,19 @@ int cgroup_task_count(const struct cgrou >  int cgroup_is_descendant(const struct cgroup *cgrp, struct task_struct *task); > >  /* > + * Allow to use CGRP_WAIT_ON_RMDIR flag to check race with rmdir() for subsys. > + * Subsys can call this function if it's necessary to call pre_destroy() again > + * because it adds not-temporary refs to css after or while pre_destroy(). > + * The caller of this function should use css_tryget(), too. > + */ > +void __cgroup_wakeup_rmdir_waiters(void); > +static inline void cgroup_wakeup_rmdir_waiters(struct cgroup *cgrp) > +{ > +       if (unlikely(test_and_clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags))) > +               __cgroup_wakeup_rmdir_waiters(); > +} > + > +/* >  * Control Group subsystem type. >  * See Documentation/cgroups/cgroups.txt for details >  */ > Index: mmotm-2.6.31-Jun25/kernel/cgroup.c > =================================================================== > --- mmotm-2.6.31-Jun25.orig/kernel/cgroup.c > +++ mmotm-2.6.31-Jun25/kernel/cgroup.c > @@ -734,14 +734,13 @@ static void cgroup_d_remove_dir(struct d >  * reference to css->refcnt. In general, this refcnt is expected to goes down >  * to zero, soon. >  * > - * CGRP_WAIT_ON_RMDIR flag is modified under cgroup's inode->i_mutex; > + * CGRP_WAIT_ON_RMDIR flag is set under cgroup's inode->i_mutex; >  */ >  DECLARE_WAIT_QUEUE_HEAD(cgroup_rmdir_waitq); > > -static void cgroup_wakeup_rmdir_waiters(const struct cgroup *cgrp) > +void __cgroup_wakeup_rmdir_waiters(void) >  { Maybe we should name this wakeup_rmdir_waiter() to emphasise that fact that there will only be one waiter (the thread doing the rmdir). > -       if (unlikely(test_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags))) > -               wake_up_all(&cgroup_rmdir_waitq); > +       wake_up_all(&cgroup_rmdir_waitq); >  } > >  static int rebind_subsystems(struct cgroupfs_root *root, > @@ -2696,33 +2695,40 @@ again: >        mutex_unlock(&cgroup_mutex); > >        /* > +        * css_put/get is provided for subsys to grab refcnt to css. In typical > +        * case, subsystem has no reference after pre_destroy(). But, under > +        * hierarchy management, some *temporal* refcnt can be hold. This sentence needs improvement/clarification. (Yes, I know it's just copied from later in the file, but it wasn't clear there either :-) ) > +        * To avoid returning -EBUSY to a user, waitqueue is used. If subsys > +        * is really busy, it should return -EBUSY at pre_destroy(). wake_up > +        * is called when css_put() is called and refcnt goes down to 0. > +        * And this WAIT_ON_RMDIR flag is cleared when subsys detect a race > +        * condition under pre_destroy()->rmdir. What exactly do you mean by pre_destroy()->rmdir ? > If flag is cleared, we need > +        * to call pre_destroy(), again. > +        */ > +       set_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags); > + > +       /* >         * Call pre_destroy handlers of subsys. Notify subsystems >         * that rmdir() request comes. >         */ >        ret = cgroup_call_pre_destroy(cgrp); > -       if (ret) > +       if (ret) { > +               clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags); >                return ret; > +       } > >        mutex_lock(&cgroup_mutex); >        parent = cgrp->parent; >        if (atomic_read(&cgrp->count) || !list_empty(&cgrp->children)) { > +               clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags); >                mutex_unlock(&cgroup_mutex); >                return -EBUSY; >        } > -       /* > -        * css_put/get is provided for subsys to grab refcnt to css. In typical > -        * case, subsystem has no reference after pre_destroy(). But, under > -        * hierarchy management, some *temporal* refcnt can be hold. > -        * To avoid returning -EBUSY to a user, waitqueue is used. If subsys > -        * is really busy, it should return -EBUSY at pre_destroy(). wake_up > -        * is called when css_put() is called and refcnt goes down to 0. > -        */ > -       set_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags); >        prepare_to_wait(&cgroup_rmdir_waitq, &wait, TASK_INTERRUPTIBLE); > - >        if (!cgroup_clear_css_refs(cgrp)) { >                mutex_unlock(&cgroup_mutex); > -               schedule(); > +               if (test_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags)) > +                       schedule(); The need for this test_bit() should be commented, I think - at first sight it doesn't seem necessary since if we've already been woken up, then the schedule() is logically a no-op anyway. We only need it because we are worried about the case where someone calls wakeup_rmdir_waiters() prior to the prepare_to_wait() > =================================================================== > --- mmotm-2.6.31-Jun25.orig/mm/memcontrol.c > +++ mmotm-2.6.31-Jun25/mm/memcontrol.c > @@ -1234,6 +1234,12 @@ static int mem_cgroup_move_account(struc >        ret = 0; >  out: >        unlock_page_cgroup(pc); > +       /* > +        * We charges against "to" which may not have any tasks. Then, "to" > +        * can be under rmdir(). But in current implementation, caller of > +        * this function is just force_empty() and it's garanteed that > +        * "to" is never removed. So, we don't check rmdir status here. > +        */ >        return ret; >  } > > @@ -1455,6 +1461,7 @@ __mem_cgroup_commit_charge_swapin(struct >                return; >        if (!ptr) >                return; > +       css_get(&ptr->css); >        pc = lookup_page_cgroup(page); >        mem_cgroup_lru_del_before_commit_swapcache(page); >        __mem_cgroup_commit_charge(ptr, pc, ctype); > @@ -1484,7 +1491,13 @@ __mem_cgroup_commit_charge_swapin(struct >                } >                rcu_read_unlock(); >        } > -       /* add this page(page_cgroup) to the LRU we want. */ > +       /* > +        * At swapin, we may charge account against cgroup which has no tasks. > +        * So, rmdir()->pre_destroy() can be called while we do this charge. > +        * In that case, we need to call pre_destroy() again. check it here. > +        */ > +       cgroup_wakeup_rmdir_waiters(ptr->css.cgroup); > +       css_put(&ptr->css); > >  } > > @@ -1691,7 +1704,7 @@ void mem_cgroup_end_migration(struct mem > >        if (!mem) >                return; > - > +       css_get(&mem->css); >        /* at migration success, oldpage->mapping is NULL. */ >        if (oldpage->mapping) { >                target = oldpage; > @@ -1731,6 +1744,14 @@ void mem_cgroup_end_migration(struct mem >         */ >        if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED) >                mem_cgroup_uncharge_page(target); > +       /* > +        * At migration, we may charge account against cgroup which has no tasks > +        * So, rmdir()->pre_destroy() can be called while we do this charge. > +        * In that case, we need to call pre_destroy() again. check it here. > +        */ > +       cgroup_wakeup_rmdir_waiters(mem->css.cgroup); > +       css_put(&mem->css); Having to do an extra get/put purely in order to seems unfortunate - is that purely to force the cgroup_clear_css_refs() to fail? Maybe we could wrap the get and the wakeup/put inside functions named "cgroup_exclude_rmdir()" "cgroup_release_rmdir()" so that the mm cgroup code is more self-explanatory. Paul