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 Content-Length: 9244 Lines: 227 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/