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Date: Wed, 30 Apr 2014 15:24:51 +0200
From: Michal Hocko <mhocko@...e.cz>
To: Tejun Heo <tj@...nel.org>
Cc: lizefan@...wei.com, cgroups@...r.kernel.org,
linux-kernel@...r.kernel.org, hannes@...xchg.org,
nasa4836@...il.com
Subject: Re: [PATCH 6/6] cgroup, memcg: implement css->id and convert
css_from_id() to use it
On Thu 24-04-14 17:02:13, Tejun Heo wrote:
> Until now, cgroup->id has been used to identify all the associated
> csses and css_from_id() takes cgroup ID and returns the matching css
> by looking up the cgroup and then dereferencing the css associated
> with it; however, now that the lifetimes of cgroup and css are
> separate, this is incorrect and breaks on the unified hierarchy when a
> controller is disabled and enabled back again before the previous
> instance is released.
>
> This patch adds css->id which is a subsystem-unique ID and converts
> css_from_id() to look up by the new css->id instead. memcg is the
> only user of css_from_id() and also converted to use css->id instead.
>
> For traditional hierarchies, this shouldn't make any functional
> difference.
>
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: Jianyu Zhan <nasa4836@...il.com>
Looks good to me
Acked-by: Michal Hocko <mhocko@...e.cz>
> ---
> include/linux/cgroup.h | 9 ++++++++
> kernel/cgroup.c | 59 ++++++++++++++++++++++++++++++++------------------
> mm/memcontrol.c | 4 ++--
> 3 files changed, 49 insertions(+), 23 deletions(-)
>
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 793f70a..2dfabb3 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -62,6 +62,12 @@ struct cgroup_subsys_state {
> /* the parent css */
> struct cgroup_subsys_state *parent;
>
> + /*
> + * Subsys-unique ID. 0 is unused and root is always 1. The
> + * matching css can be looked up using css_from_id().
> + */
> + int id;
> +
> unsigned int flags;
>
> /* percpu_ref killing and RCU release */
> @@ -655,6 +661,9 @@ struct cgroup_subsys {
> /* link to parent, protected by cgroup_lock() */
> struct cgroup_root *root;
>
> + /* idr for css->id */
> + struct idr css_idr;
> +
> /*
> * List of cftypes. Each entry is the first entry of an array
> * terminated by zero length name.
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index f1c98c5..a1a20e8 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -100,8 +100,8 @@ static DECLARE_RWSEM(css_set_rwsem);
> #endif
>
> /*
> - * Protects cgroup_idr so that IDs can be released without grabbing
> - * cgroup_mutex.
> + * Protects cgroup_idr and css_idr so that IDs can be released without
> + * grabbing cgroup_mutex.
> */
> static DEFINE_SPINLOCK(cgroup_idr_lock);
>
> @@ -1089,12 +1089,6 @@ static void cgroup_put(struct cgroup *cgrp)
> if (WARN_ON_ONCE(cgrp->parent && !cgroup_is_dead(cgrp)))
> return;
>
> - /*
> - * XXX: cgrp->id is only used to look up css's. As cgroup and
> - * css's lifetimes will be decoupled, it should be made
> - * per-subsystem and moved to css->id so that lookups are
> - * successful until the target css is released.
> - */
> cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
> cgrp->id = -1;
>
> @@ -4104,8 +4098,11 @@ static void css_release(struct percpu_ref *ref)
> {
> struct cgroup_subsys_state *css =
> container_of(ref, struct cgroup_subsys_state, refcnt);
> + struct cgroup_subsys *ss = css->ss;
> +
> + RCU_INIT_POINTER(css->cgroup->subsys[ss->id], NULL);
> + cgroup_idr_remove(&ss->css_idr, css->id);
>
> - RCU_INIT_POINTER(css->cgroup->subsys[css->ss->id], NULL);
> call_rcu(&css->rcu_head, css_free_rcu_fn);
> }
>
> @@ -4195,9 +4192,17 @@ static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss)
> if (err)
> goto err_free_css;
>
> + err = cgroup_idr_alloc(&ss->css_idr, NULL, 2, 0, GFP_NOWAIT);
> + if (err < 0)
> + goto err_free_percpu_ref;
> + css->id = err;
> +
> err = cgroup_populate_dir(cgrp, 1 << ss->id);
> if (err)
> - goto err_free_percpu_ref;
> + goto err_free_id;
> +
> + /* @css is ready to be brought online now, make it visible */
> + cgroup_idr_replace(&ss->css_idr, css, css->id);
>
> err = online_css(css);
> if (err)
> @@ -4216,6 +4221,8 @@ static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss)
>
> err_clear_dir:
> cgroup_clear_dir(css->cgroup, 1 << css->ss->id);
> +err_free_id:
> + cgroup_idr_remove(&ss->css_idr, css->id);
> err_free_percpu_ref:
> percpu_ref_cancel_init(&css->refcnt);
> err_free_css:
> @@ -4642,7 +4649,7 @@ static struct kernfs_syscall_ops cgroup_kf_syscall_ops = {
> .rename = cgroup_rename,
> };
>
> -static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
> +static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
> {
> struct cgroup_subsys_state *css;
>
> @@ -4651,6 +4658,7 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
> mutex_lock(&cgroup_tree_mutex);
> mutex_lock(&cgroup_mutex);
>
> + idr_init(&ss->css_idr);
> INIT_LIST_HEAD(&ss->cfts);
>
> /* Create the root cgroup state for this subsystem */
> @@ -4659,6 +4667,13 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
> /* We don't handle early failures gracefully */
> BUG_ON(IS_ERR(css));
> init_and_link_css(css, ss, &cgrp_dfl_root.cgrp);
> + if (early) {
> + /* idr_alloc() can't be called safely during early init */
> + css->id = 1;
> + } else {
> + css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2, GFP_KERNEL);
> + BUG_ON(css->id < 0);
> + }
>
> /* Update the init_css_set to contain a subsys
> * pointer to this state - since the subsystem is
> @@ -4709,7 +4724,7 @@ int __init cgroup_init_early(void)
> ss->name = cgroup_subsys_name[i];
>
> if (ss->early_init)
> - cgroup_init_subsys(ss);
> + cgroup_init_subsys(ss, true);
> }
> return 0;
> }
> @@ -4741,8 +4756,16 @@ int __init cgroup_init(void)
> mutex_unlock(&cgroup_tree_mutex);
>
> for_each_subsys(ss, ssid) {
> - if (!ss->early_init)
> - cgroup_init_subsys(ss);
> + if (ss->early_init) {
> + struct cgroup_subsys_state *css =
> + init_css_set.subsys[ss->id];
> +
> + css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2,
> + GFP_KERNEL);
> + BUG_ON(css->id < 0);
> + } else {
> + cgroup_init_subsys(ss, false);
> + }
>
> list_add_tail(&init_css_set.e_cset_node[ssid],
> &cgrp_dfl_root.cgrp.e_csets[ssid]);
> @@ -5196,14 +5219,8 @@ struct cgroup_subsys_state *css_tryget_from_dir(struct dentry *dentry,
> */
> struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)
> {
> - struct cgroup *cgrp;
> -
> WARN_ON_ONCE(!rcu_read_lock_held());
> -
> - cgrp = idr_find(&ss->root->cgroup_idr, id);
> - if (cgrp)
> - return cgroup_css(cgrp, ss);
> - return NULL;
> + return idr_find(&ss->css_idr, id);
> }
>
> #ifdef CONFIG_CGROUP_DEBUG
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 1d0b297..c3f82f6 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -527,7 +527,7 @@ static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
>
> static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
> {
> - return memcg->css.cgroup->id;
> + return memcg->css.id;
> }
>
> static inline struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
> @@ -6401,7 +6401,7 @@ mem_cgroup_css_online(struct cgroup_subsys_state *css)
> struct mem_cgroup *memcg = mem_cgroup_from_css(css);
> struct mem_cgroup *parent = mem_cgroup_from_css(css_parent(css));
>
> - if (css->cgroup->id > MEM_CGROUP_ID_MAX)
> + if (css->id > MEM_CGROUP_ID_MAX)
> return -ENOSPC;
>
> if (!parent)
> --
> 1.9.0
>
--
Michal Hocko
SUSE Labs
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