From 05946a1e0fdb011ac0e6638ee60b181c584f127b Mon Sep 17 00:00:00 2001 From: Colin Cross Date: Tue, 23 Nov 2010 21:37:04 -0800 Subject: [PATCH] cgroup: Remove call to synchronize_rcu in cgroup_attach_task synchronize_rcu can be very expensive, averaging 100 ms in some cases. In cgroup_attach_task, it is used to prevent a task->cgroups pointer dereferenced in an RCU read side critical section from being invalidated, by delaying the call to put_css_set until after an RCU grace period. To avoid the call to synchronize_rcu, make the put_css_set call rcu-safe by moving the deletion of the css_set links into free_css_set_work, scheduled by the rcu callback free_css_set_rcu. The decrement of the cgroup refcount is no longer synchronous with the call to put_css_set, which can result in the cgroup refcount staying positive after the last call to cgroup_attach_task returns. To allow the cgroup to be deleted with cgroup_rmdir synchronously after cgroup_attach_task, have rmdir check the refcount of all associated css_sets. If cgroup_rmdir is called on a cgroup for which the css_sets all have refcount zero but the cgroup refcount is nonzero, reuse the rmdir waitqueue to block the rmdir until free_css_set_work is called. Signed-off-by: Colin Cross --- include/linux/cgroup.h | 1 + kernel/cgroup.c | 120 +++++++++++++++++++++++++---------------- 2 files changed, 74 insertions(+), 47 deletions(-) diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h index 8296e6b9f15d..ae9613be93fd 100644 --- a/include/linux/cgroup.h +++ b/include/linux/cgroup.h @@ -275,6 +275,7 @@ struct css_set { /* For RCU-protected deletion */ struct rcu_head rcu_head; + struct work_struct work; }; /* diff --git a/kernel/cgroup.c b/kernel/cgroup.c index c2ba4e9c7f95..3f1552c7ee11 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -264,6 +264,33 @@ static void cgroup_release_agent(struct work_struct *work); static DECLARE_WORK(release_agent_work, cgroup_release_agent); static void check_for_release(struct cgroup *cgrp); +/* + * A queue for waiters to do rmdir() cgroup. A tasks will sleep when + * cgroup->count == 0 && list_empty(&cgroup->children) && subsys has some + * reference to css->refcnt. In general, this refcnt is expected to goes down + * to zero, soon. + * + * 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_waiter(struct cgroup *cgrp) +{ + if (unlikely(test_and_clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags))) + wake_up_all(&cgroup_rmdir_waitq); +} + +void cgroup_exclude_rmdir(struct cgroup_subsys_state *css) +{ + css_get(css); +} + +void cgroup_release_and_wakeup_rmdir(struct cgroup_subsys_state *css) +{ + cgroup_wakeup_rmdir_waiter(css->cgroup); + css_put(css); +} + /* Link structure for associating css_set objects with cgroups */ struct cg_cgroup_link { /* @@ -323,10 +350,35 @@ static struct hlist_head *css_set_hash(struct cgroup_subsys_state *css[]) return &css_set_table[index]; } +static void free_css_set_work(struct work_struct *work) +{ + struct css_set *cg = container_of(work, struct css_set, work); + struct cg_cgroup_link *link; + struct cg_cgroup_link *saved_link; + + write_lock(&css_set_lock); + list_for_each_entry_safe(link, saved_link, &cg->cg_links, + cg_link_list) { + struct cgroup *cgrp = link->cgrp; + list_del(&link->cg_link_list); + list_del(&link->cgrp_link_list); + if (atomic_dec_and_test(&cgrp->count)) { + check_for_release(cgrp); + cgroup_wakeup_rmdir_waiter(cgrp); + } + kfree(link); + } + write_unlock(&css_set_lock); + + kfree(cg); +} + static void free_css_set_rcu(struct rcu_head *obj) { struct css_set *cg = container_of(obj, struct css_set, rcu_head); - kfree(cg); + + INIT_WORK(&cg->work, free_css_set_work); + schedule_work(&cg->work); } /* We don't maintain the lists running through each css_set to its @@ -345,8 +397,6 @@ static inline void get_css_set(struct css_set *cg) static void put_css_set(struct css_set *cg) { - struct cg_cgroup_link *link; - struct cg_cgroup_link *saved_link; /* * Ensure that the refcount doesn't hit zero while any readers * can see it. Similar to atomic_dec_and_lock(), but for an @@ -360,21 +410,9 @@ static void put_css_set(struct css_set *cg) return; } - /* This css_set is dead. unlink it and release cgroup refcounts */ hlist_del(&cg->hlist); css_set_count--; - list_for_each_entry_safe(link, saved_link, &cg->cg_links, - cg_link_list) { - struct cgroup *cgrp = link->cgrp; - list_del(&link->cg_link_list); - list_del(&link->cgrp_link_list); - if (atomic_dec_and_test(&cgrp->count)) - check_for_release(cgrp); - - kfree(link); - } - write_unlock(&css_set_lock); call_rcu(&cg->rcu_head, free_css_set_rcu); } @@ -708,9 +746,9 @@ static struct cgroup *task_cgroup_from_root(struct task_struct *task, * cgroup_attach_task(), which overwrites one tasks cgroup pointer with * another. It does so using cgroup_mutex, however there are * several performance critical places that need to reference - * task->cgroup without the expense of grabbing a system global + * task->cgroups without the expense of grabbing a system global * mutex. Therefore except as noted below, when dereferencing or, as - * in cgroup_attach_task(), modifying a task'ss cgroup pointer we use + * in cgroup_attach_task(), modifying a task's cgroups pointer we use * task_lock(), which acts on a spinlock (task->alloc_lock) already in * the task_struct routinely used for such matters. * @@ -890,33 +928,6 @@ static void cgroup_d_remove_dir(struct dentry *dentry) remove_dir(dentry); } -/* - * A queue for waiters to do rmdir() cgroup. A tasks will sleep when - * cgroup->count == 0 && list_empty(&cgroup->children) && subsys has some - * reference to css->refcnt. In general, this refcnt is expected to goes down - * to zero, soon. - * - * 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_waiter(struct cgroup *cgrp) -{ - if (unlikely(test_and_clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags))) - wake_up_all(&cgroup_rmdir_waitq); -} - -void cgroup_exclude_rmdir(struct cgroup_subsys_state *css) -{ - css_get(css); -} - -void cgroup_release_and_wakeup_rmdir(struct cgroup_subsys_state *css) -{ - cgroup_wakeup_rmdir_waiter(css->cgroup); - css_put(css); -} - /* * Call with cgroup_mutex held. Drops reference counts on modules, including * any duplicate ones that parse_cgroupfs_options took. If this function @@ -1760,7 +1771,7 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk) ss->attach(ss, cgrp, oldcgrp, tsk, false); } set_bit(CGRP_RELEASABLE, &cgrp->flags); - synchronize_rcu(); + /* put_css_set will not destroy cg until after an RCU grace period */ put_css_set(cg); /* @@ -3480,6 +3491,21 @@ static int cgroup_clear_css_refs(struct cgroup *cgrp) return !failed; } +/* checks if all of the css_sets attached to a cgroup have a refcount of 0. + * Must be called with css_set_lock held */ +static int cgroup_css_sets_empty(struct cgroup *cgrp) +{ + struct cg_cgroup_link *link; + + list_for_each_entry(link, &cgrp->css_sets, cgrp_link_list) { + struct css_set *cg = link->cg; + if (atomic_read(&cg->refcount) > 0) + return 0; + } + + return 1; +} + static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry) { struct cgroup *cgrp = dentry->d_fsdata; @@ -3492,7 +3518,7 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry) /* the vfs holds both inode->i_mutex already */ again: mutex_lock(&cgroup_mutex); - if (atomic_read(&cgrp->count) != 0) { + if (!cgroup_css_sets_empty(cgrp)) { mutex_unlock(&cgroup_mutex); return -EBUSY; } @@ -3525,7 +3551,7 @@ again: mutex_lock(&cgroup_mutex); parent = cgrp->parent; - if (atomic_read(&cgrp->count) || !list_empty(&cgrp->children)) { + if (!cgroup_css_sets_empty(cgrp) || !list_empty(&cgrp->children)) { clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags); mutex_unlock(&cgroup_mutex); return -EBUSY; -- 2.34.1