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Message-ID: <20081211062940.GJ3008@balbir.in.ibm.com>
Date: Thu, 11 Dec 2008 11:59:40 +0530
From: Balbir Singh <balbir@...ux.vnet.ibm.com>
To: menage@...gle.com
Cc: kamezawa.hiroyu@...fujitsu.com,
containers@...ts.linux-foundation.org,
linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [RFC][PATCH 1/3] CGroups: Add a per-subsystem hierarchy_mutex
* menage@...gle.com <menage@...gle.com> [2008-12-10 15:36:55]:
> This patch adds a hierarchy_mutex to the cgroup_subsys object that
> protects changes to the hierarchy observed by that subsystem. It is
> taken by the cgroup subsystem (in addition to cgroup_mutex) for the
> following operations:
>
> - linking a cgroup into that subsystem's cgroup tree
> - unlinking a cgroup from that subsystem's cgroup tree
> - moving the subsystem to/from a hierarchy (including across the
> bind() callback)
>
> Thus if the subsystem holds its own hierarchy_mutex, it can safely
> traverse its own hierarchy.
>
Ths sounds reasonable, a further abstraction in the future could be to
provide the visitor pattern. Allow cgroups to do the walking and have
callbacks called during the visit.
> Signed-off-by: Paul Menage <menage@...gle.com>
>
> ---
>
> Documentation/cgroups/cgroups.txt | 2 +-
> include/linux/cgroup.h | 9 ++++++++-
> kernel/cgroup.c | 37 +++++++++++++++++++++++++++++++++++--
> 3 files changed, 44 insertions(+), 4 deletions(-)
>
> Index: hierarchy_lock-mmotm-2008-12-09/include/linux/cgroup.h
> ===================================================================
> --- hierarchy_lock-mmotm-2008-12-09.orig/include/linux/cgroup.h
> +++ hierarchy_lock-mmotm-2008-12-09/include/linux/cgroup.h
> @@ -337,8 +337,15 @@ struct cgroup_subsys {
> #define MAX_CGROUP_TYPE_NAMELEN 32
> const char *name;
>
> - struct cgroupfs_root *root;
> + /*
> + * Protects sibling/children links of cgroups in this
> + * hierarchy, plus protects which hierarchy (or none) the
> + * subsystem is a part of (i.e. root/sibling)
> + */
> + struct mutex hierarchy_mutex;
>
> + /* Protected by this->hierarchy_mutex and cgroup_lock() */
> + struct cgroupfs_root *root;
> struct list_head sibling;
> };
>
> Index: hierarchy_lock-mmotm-2008-12-09/kernel/cgroup.c
> ===================================================================
> --- hierarchy_lock-mmotm-2008-12-09.orig/kernel/cgroup.c
> +++ hierarchy_lock-mmotm-2008-12-09/kernel/cgroup.c
> @@ -714,23 +714,26 @@ static int rebind_subsystems(struct cgro
> BUG_ON(cgrp->subsys[i]);
> BUG_ON(!dummytop->subsys[i]);
> BUG_ON(dummytop->subsys[i]->cgroup != dummytop);
> + mutex_lock(&ss->hierarchy_mutex);
> cgrp->subsys[i] = dummytop->subsys[i];
> cgrp->subsys[i]->cgroup = cgrp;
> list_move(&ss->sibling, &root->subsys_list);
> ss->root = root;
> if (ss->bind)
> ss->bind(ss, cgrp);
> -
> + mutex_unlock(&ss->hierarchy_mutex);
> } else if (bit & removed_bits) {
> /* We're removing this subsystem */
> BUG_ON(cgrp->subsys[i] != dummytop->subsys[i]);
> BUG_ON(cgrp->subsys[i]->cgroup != cgrp);
> + mutex_lock(&ss->hierarchy_mutex);
> if (ss->bind)
> ss->bind(ss, dummytop);
> dummytop->subsys[i]->cgroup = dummytop;
> cgrp->subsys[i] = NULL;
> subsys[i]->root = &rootnode;
> list_move(&ss->sibling, &rootnode.subsys_list);
> + mutex_unlock(&ss->hierarchy_mutex);
> } else if (bit & final_bits) {
> /* Subsystem state should already exist */
> BUG_ON(!cgrp->subsys[i]);
> @@ -2326,6 +2329,29 @@ static void init_cgroup_css(struct cgrou
> cgrp->subsys[ss->subsys_id] = css;
> }
>
> +static void cgroup_lock_hierarchy(struct cgroupfs_root *root)
> +{
> + /* We need to take each hierarchy_mutex in a consistent order */
> + int i;
> +
> + for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
> + struct cgroup_subsys *ss = subsys[i];
> + if (ss->root == root)
> + mutex_lock_nested(&ss->hierarchy_mutex, i);
> + }
> +}
> +
> +static void cgroup_unlock_hierarchy(struct cgroupfs_root *root)
> +{
> + int i;
> +
> + for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
> + struct cgroup_subsys *ss = subsys[i];
> + if (ss->root == root)
> + mutex_unlock(&ss->hierarchy_mutex);
> + }
> +}
> +
> /*
> * cgroup_create - create a cgroup
> * @parent: cgroup that will be parent of the new cgroup
> @@ -2374,7 +2400,9 @@ static long cgroup_create(struct cgroup
> init_cgroup_css(css, ss, cgrp);
> }
>
> + cgroup_lock_hierarchy(root);
> list_add(&cgrp->sibling, &cgrp->parent->children);
> + cgroup_unlock_hierarchy(root);
> root->number_of_cgroups++;
>
> err = cgroup_create_dir(cgrp, dentry, mode);
> @@ -2492,8 +2520,12 @@ static int cgroup_rmdir(struct inode *un
> if (!list_empty(&cgrp->release_list))
> list_del(&cgrp->release_list);
> spin_unlock(&release_list_lock);
> - /* delete my sibling from parent->children */
> +
> + cgroup_lock_hierarchy(cgrp->root);
> + /* delete this cgroup from parent->children */
> list_del(&cgrp->sibling);
> + cgroup_unlock_hierarchy(cgrp->root);
> +
> spin_lock(&cgrp->dentry->d_lock);
> d = dget(cgrp->dentry);
> spin_unlock(&d->d_lock);
> @@ -2535,6 +2567,7 @@ static void __init cgroup_init_subsys(st
> * need to invoke fork callbacks here. */
> BUG_ON(!list_empty(&init_task.tasks));
>
> + mutex_init(&ss->hierarchy_mutex);
> ss->active = 1;
> }
>
> Index: hierarchy_lock-mmotm-2008-12-09/Documentation/cgroups/cgroups.txt
> ===================================================================
> --- hierarchy_lock-mmotm-2008-12-09.orig/Documentation/cgroups/cgroups.txt
> +++ hierarchy_lock-mmotm-2008-12-09/Documentation/cgroups/cgroups.txt
> @@ -528,7 +528,7 @@ example in cpusets, no task may attach b
> up.
>
> void bind(struct cgroup_subsys *ss, struct cgroup *root)
> -(cgroup_mutex held by caller)
> +(cgroup_mutex and ss->hierarchy_mutex held by caller)
>
Seems reasonable, I was wondering if instead of acquiring a mutex per
subsystem that shares the root, if we can collapse it to a single
mutex and prevent cgroup from changing binding. Those are
optimizations that we can think of later
Acked-by: Balbir Singh <balbir@...ux.vnet.ibm.com>
--
Balbir
--
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