Re: [PATCH 2/2] cgroup: Remove call to synchronize_rcu in cgroup_attach_task

[Bryan Huntsman]

On 11/23/2010 09:37 PM, Colin Cross wrote:
> 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 <ccross@xxxxxxxxxxx>
> ---
>  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 9e13078..49fdff0 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -279,6 +279,7 @@ 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 34e855e..e752c83 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -267,6 +267,33 @@ 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 {
>  	/*
> @@ -326,10 +353,35 @@ 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
> @@ -348,8 +400,6 @@ 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
> @@ -363,21 +413,9 @@ 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);
>  }
> @@ -711,9 +749,9 @@ 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.
>   *
> @@ -895,33 +933,6 @@ static void cgroup_d_remove_dir(struct dentry *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
>   * returns an error, no reference counts are touched.
> @@ -1788,7 +1799,7 @@ 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);
>  
>  	/*
> @@ -3546,6 +3557,21 @@ 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;
> @@ -3558,7 +3584,7 @@ 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;
>  	}
> @@ -3591,7 +3617,7 @@ 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;

Tested-by: Mike Bohan <mbohan@xxxxxxxxxxxxxx>

I'm responding on Mike's behalf and adding him to this thread.  This
patch improves launch time of a test app from ~700ms to ~250ms on MSM,
with much lower variance across tests.  We also see UI latency
improvements, but have not quantified the gains.

- Bryan

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
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