Re: [PATCH 1/2] cgroup: Set CGRP_RELEASABLE when adding to a cgroup

[Bryan Huntsman <bryanh@codeaurora.org>]

On 11/23/2010 09:37 PM, Colin Cross wrote:
> Changes the meaning of CGRP_RELEASABLE to be set on any cgroup
> that has ever had a task or cgroup in it, or had css_get called
> on it.  The bit is set in cgroup_attach_task, cgroup_create,
> and __css_get.  It is not necessary to set the bit in
> cgroup_fork, as the task is either in the root cgroup, in
> which can never be released, or the task it was forked from
> already set the bit in croup_attach_task.
> 
> Signed-off-by: Colin Cross <ccross@android.com>
> ---
>  include/linux/cgroup.h |   12 +--------
>  kernel/cgroup.c        |   54 ++++++++++++++++++++---------------------------
>  2 files changed, 25 insertions(+), 41 deletions(-)
> 
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index ed4ba11..9e13078 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -84,12 +84,6 @@ enum {
>  	CSS_REMOVED, /* This CSS is dead */
>  };
>  
> -/* Caller must verify that the css is not for root cgroup */
> -static inline void __css_get(struct cgroup_subsys_state *css, int count)
> -{
> -	atomic_add(count, &css->refcnt);
> -}
> -
>  /*
>   * Call css_get() to hold a reference on the css; it can be used
>   * for a reference obtained via:
> @@ -97,6 +91,7 @@ static inline void __css_get(struct cgroup_subsys_state *css, int count)
>   * - task->cgroups for a locked task
>   */
>  
> +extern void __css_get(struct cgroup_subsys_state *css, int count);
>  static inline void css_get(struct cgroup_subsys_state *css)
>  {
>  	/* We don't need to reference count the root state */
> @@ -143,10 +138,7 @@ static inline void css_put(struct cgroup_subsys_state *css)
>  enum {
>  	/* Control Group is dead */
>  	CGRP_REMOVED,
> -	/*
> -	 * Control Group has previously had a child cgroup or a task,
> -	 * but no longer (only if CGRP_NOTIFY_ON_RELEASE is set)
> -	 */
> +	/* Control Group has ever had a child cgroup or a task */
>  	CGRP_RELEASABLE,
>  	/* Control Group requires release notifications to userspace */
>  	CGRP_NOTIFY_ON_RELEASE,
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 66a416b..34e855e 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -338,7 +338,15 @@ static void free_css_set_rcu(struct rcu_head *obj)
>   * compiled into their kernel but not actually in use */
>  static int use_task_css_set_links __read_mostly;
>  
> -static void __put_css_set(struct css_set *cg, int taskexit)
> +/*
> + * refcounted get/put for css_set objects
> + */
> +static inline void get_css_set(struct css_set *cg)
> +{
> +	atomic_inc(&cg->refcount);
> +}
> +
> +static void put_css_set(struct css_set *cg)
>  {
>  	struct cg_cgroup_link *link;
>  	struct cg_cgroup_link *saved_link;
> @@ -364,12 +372,8 @@ static void __put_css_set(struct css_set *cg, int taskexit)
>  		struct cgroup *cgrp = link->cgrp;
>  		list_del(&link->cg_link_list);
>  		list_del(&link->cgrp_link_list);
> -		if (atomic_dec_and_test(&cgrp->count) &&
> -		    notify_on_release(cgrp)) {
> -			if (taskexit)
> -				set_bit(CGRP_RELEASABLE, &cgrp->flags);
> +		if (atomic_dec_and_test(&cgrp->count))
>  			check_for_release(cgrp);
> -		}
>  
>  		kfree(link);
>  	}
> @@ -379,24 +383,6 @@ static void __put_css_set(struct css_set *cg, int taskexit)
>  }
>  
>  /*
> - * refcounted get/put for css_set objects
> - */
> -static inline void get_css_set(struct css_set *cg)
> -{
> -	atomic_inc(&cg->refcount);
> -}
> -
> -static inline void put_css_set(struct css_set *cg)
> -{
> -	__put_css_set(cg, 0);
> -}
> -
> -static inline void put_css_set_taskexit(struct css_set *cg)
> -{
> -	__put_css_set(cg, 1);
> -}
> -
> -/*
>   * compare_css_sets - helper function for find_existing_css_set().
>   * @cg: candidate css_set being tested
>   * @old_cg: existing css_set for a task
> @@ -1801,7 +1787,7 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
>  		if (ss->attach)
>  			ss->attach(ss, cgrp, oldcgrp, tsk, false);
>  	}
> -	set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
> +	set_bit(CGRP_RELEASABLE, &cgrp->flags);
>  	synchronize_rcu();
>  	put_css_set(cg);
>  
> @@ -3427,6 +3413,8 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
>  	if (err < 0)
>  		goto err_remove;
>  
> +	set_bit(CGRP_RELEASABLE, &parent->flags);
> +
>  	/* The cgroup directory was pre-locked for us */
>  	BUG_ON(!mutex_is_locked(&cgrp->dentry->d_inode->i_mutex));
>  
> @@ -3645,7 +3633,6 @@ again:
>  	cgroup_d_remove_dir(d);
>  	dput(d);
>  
> -	set_bit(CGRP_RELEASABLE, &parent->flags);
>  	check_for_release(parent);
>  
>  	/*
> @@ -4240,7 +4227,7 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
>  	tsk->cgroups = &init_css_set;
>  	task_unlock(tsk);
>  	if (cg)
> -		put_css_set_taskexit(cg);
> +		put_css_set(cg);
>  }
>  
>  /**
> @@ -4410,6 +4397,14 @@ static void check_for_release(struct cgroup *cgrp)
>  }
>  
>  /* Caller must verify that the css is not for root cgroup */
> +void __css_get(struct cgroup_subsys_state *css, int count)
> +{
> +	atomic_add(count, &css->refcnt);
> +	set_bit(CGRP_RELEASABLE, &css->cgroup->flags);
> +}
> +EXPORT_SYMBOL_GPL(__css_get);
> +
> +/* Caller must verify that the css is not for root cgroup */
>  void __css_put(struct cgroup_subsys_state *css, int count)
>  {
>  	struct cgroup *cgrp = css->cgroup;
> @@ -4417,10 +4412,7 @@ void __css_put(struct cgroup_subsys_state *css, int count)
>  	rcu_read_lock();
>  	val = atomic_sub_return(count, &css->refcnt);
>  	if (val == 1) {
> -		if (notify_on_release(cgrp)) {
> -			set_bit(CGRP_RELEASABLE, &cgrp->flags);
> -			check_for_release(cgrp);
> -		}
> +		check_for_release(cgrp);
>  		cgroup_wakeup_rmdir_waiter(cgrp);
>  	}
>  	rcu_read_unlock();

Tested-by: Mike Bohan <mbohan@codeaurora.org>

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.