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cgroup: remove extra calls to find_existing_css_set
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In cgroup_attach_proc, we indirectly call find_existing_css_set 3
times. It is an expensive call so we want to call it a minimum
of times. This patch only calls it once and stores the result so
that it can be used later on when we call cgroup_task_migrate.

This required modifying cgroup_task_migrate to take the new css_set
(which we obtained from find_css_set) as a parameter. The nice side
effect of this is that cgroup_task_migrate is now identical for
cgroup_attach_task and cgroup_attach_proc. It also now returns a
void since it can never fail.

Changes in V5:
* https://lkml.org/lkml/2012/1/20/344 (Tejun Heo)
  * Remove css_set_refs
Changes in V4:
* https://lkml.org/lkml/2011/12/22/421 (Li Zefan)
  * Avoid GFP_KERNEL (sleep) in rcu_read_lock by getting css_set in
    a separate loop not under an rcu_read_lock
Changes in V3:
* https://lkml.org/lkml/2011/12/22/13 (Li Zefan)
  * Fixed earlier bug by creating a seperate patch to remove tasklist_lock
Changes in V2:
* https://lkml.org/lkml/2011/12/20/372 (Tejun Heo)
  * Move find_css_set call into loop which creates the flex array
* Author
  * Kill css_set_refs and use group_size instead
  * Fix an off-by-one error in counting css_set refs
  * Add a retval check in out_list_teardown

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: containers@lists.linux-foundation.org
Cc: cgroups@vger.kernel.org
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <paul@paulmenage.org>
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Mandeep Singh Baines authored and Tejun Heo committed Jan 30, 2012
1 parent fb5d2b4 commit 61d1d21
Showing 1 changed file with 27 additions and 113 deletions.
140 changes: 27 additions & 113 deletions kernel/cgroup.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1763,6 +1763,7 @@ EXPORT_SYMBOL_GPL(cgroup_path);
struct task_and_cgroup {
struct task_struct *task;
struct cgroup *cgrp;
struct css_set *cg;
};

struct cgroup_taskset {
Expand Down Expand Up @@ -1843,11 +1844,10 @@ EXPORT_SYMBOL_GPL(cgroup_taskset_size);
* will already exist. If not set, this function might sleep, and can fail with
* -ENOMEM. Must be called with cgroup_mutex and threadgroup locked.
*/
static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
struct task_struct *tsk, bool guarantee)
static void cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
struct task_struct *tsk, struct css_set *newcg)
{
struct css_set *oldcg;
struct css_set *newcg;

/*
* We are synchronized through threadgroup_lock() against PF_EXITING
Expand All @@ -1857,23 +1857,6 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
WARN_ON_ONCE(tsk->flags & PF_EXITING);
oldcg = tsk->cgroups;

/* locate or allocate a new css_set for this task. */
if (guarantee) {
/* we know the css_set we want already exists. */
struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];
read_lock(&css_set_lock);
newcg = find_existing_css_set(oldcg, cgrp, template);
BUG_ON(!newcg);
get_css_set(newcg);
read_unlock(&css_set_lock);
} else {
might_sleep();
/* find_css_set will give us newcg already referenced. */
newcg = find_css_set(oldcg, cgrp);
if (!newcg)
return -ENOMEM;
}

task_lock(tsk);
rcu_assign_pointer(tsk->cgroups, newcg);
task_unlock(tsk);
Expand All @@ -1892,7 +1875,6 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
put_css_set(oldcg);

set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
return 0;
}

/**
Expand All @@ -1910,6 +1892,7 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
struct cgroup *oldcgrp;
struct cgroupfs_root *root = cgrp->root;
struct cgroup_taskset tset = { };
struct css_set *newcg;

/* @tsk either already exited or can't exit until the end */
if (tsk->flags & PF_EXITING)
Expand Down Expand Up @@ -1939,9 +1922,13 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
}
}

retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, false);
if (retval)
newcg = find_css_set(tsk->cgroups, cgrp);
if (!newcg) {
retval = -ENOMEM;
goto out;
}

cgroup_task_migrate(cgrp, oldcgrp, tsk, newcg);

for_each_subsys(root, ss) {
if (ss->attach)
Expand Down Expand Up @@ -1997,66 +1984,6 @@ int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)
}
EXPORT_SYMBOL_GPL(cgroup_attach_task_all);

/*
* cgroup_attach_proc works in two stages, the first of which prefetches all
* new css_sets needed (to make sure we have enough memory before committing
* to the move) and stores them in a list of entries of the following type.
* TODO: possible optimization: use css_set->rcu_head for chaining instead
*/
struct cg_list_entry {
struct css_set *cg;
struct list_head links;
};

static bool css_set_check_fetched(struct cgroup *cgrp,
struct task_struct *tsk, struct css_set *cg,
struct list_head *newcg_list)
{
struct css_set *newcg;
struct cg_list_entry *cg_entry;
struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];

read_lock(&css_set_lock);
newcg = find_existing_css_set(cg, cgrp, template);
read_unlock(&css_set_lock);

/* doesn't exist at all? */
if (!newcg)
return false;
/* see if it's already in the list */
list_for_each_entry(cg_entry, newcg_list, links)
if (cg_entry->cg == newcg)
return true;

/* not found */
return false;
}

/*
* Find the new css_set and store it in the list in preparation for moving the
* given task to the given cgroup. Returns 0 or -ENOMEM.
*/
static int css_set_prefetch(struct cgroup *cgrp, struct css_set *cg,
struct list_head *newcg_list)
{
struct css_set *newcg;
struct cg_list_entry *cg_entry;

/* ensure a new css_set will exist for this thread */
newcg = find_css_set(cg, cgrp);
if (!newcg)
return -ENOMEM;
/* add it to the list */
cg_entry = kmalloc(sizeof(struct cg_list_entry), GFP_KERNEL);
if (!cg_entry) {
put_css_set(newcg);
return -ENOMEM;
}
cg_entry->cg = newcg;
list_add(&cg_entry->links, newcg_list);
return 0;
}

/**
* cgroup_attach_proc - attach all threads in a threadgroup to a cgroup
* @cgrp: the cgroup to attach to
Expand All @@ -2070,20 +1997,12 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
int retval, i, group_size;
struct cgroup_subsys *ss, *failed_ss = NULL;
/* guaranteed to be initialized later, but the compiler needs this */
struct css_set *oldcg;
struct cgroupfs_root *root = cgrp->root;
/* threadgroup list cursor and array */
struct task_struct *tsk;
struct task_and_cgroup *tc;
struct flex_array *group;
struct cgroup_taskset tset = { };
/*
* we need to make sure we have css_sets for all the tasks we're
* going to move -before- we actually start moving them, so that in
* case we get an ENOMEM we can bail out before making any changes.
*/
struct list_head newcg_list;
struct cg_list_entry *cg_entry, *temp_nobe;

/*
* step 0: in order to do expensive, possibly blocking operations for
Expand Down Expand Up @@ -2119,15 +2038,15 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)

/* as per above, nr_threads may decrease, but not increase. */
BUG_ON(i >= group_size);
/*
* saying GFP_ATOMIC has no effect here because we did prealloc
* earlier, but it's good form to communicate our expectations.
*/
ent.task = tsk;
ent.cgrp = task_cgroup_from_root(tsk, root);
/* nothing to do if this task is already in the cgroup */
if (ent.cgrp == cgrp)
continue;
/*
* saying GFP_ATOMIC has no effect here because we did prealloc
* earlier, but it's good form to communicate our expectations.
*/
retval = flex_array_put(group, i, &ent, GFP_ATOMIC);
BUG_ON(retval != 0);
i++;
Expand Down Expand Up @@ -2160,17 +2079,12 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
* step 2: make sure css_sets exist for all threads to be migrated.
* we use find_css_set, which allocates a new one if necessary.
*/
INIT_LIST_HEAD(&newcg_list);
for (i = 0; i < group_size; i++) {
tc = flex_array_get(group, i);
oldcg = tc->task->cgroups;

/* if we don't already have it in the list get a new one */
if (!css_set_check_fetched(cgrp, tc->task, oldcg,
&newcg_list)) {
retval = css_set_prefetch(cgrp, oldcg, &newcg_list);
if (retval)
goto out_list_teardown;
tc->cg = find_css_set(tc->task->cgroups, cgrp);
if (!tc->cg) {
retval = -ENOMEM;
goto out_put_css_set_refs;
}
}

Expand All @@ -2181,8 +2095,7 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
*/
for (i = 0; i < group_size; i++) {
tc = flex_array_get(group, i);
retval = cgroup_task_migrate(cgrp, tc->cgrp, tc->task, true);
BUG_ON(retval);
cgroup_task_migrate(cgrp, tc->cgrp, tc->task, tc->cg);
}
/* nothing is sensitive to fork() after this point. */

Expand All @@ -2200,15 +2113,16 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
synchronize_rcu();
cgroup_wakeup_rmdir_waiter(cgrp);
retval = 0;
out_list_teardown:
/* clean up the list of prefetched css_sets. */
list_for_each_entry_safe(cg_entry, temp_nobe, &newcg_list, links) {
list_del(&cg_entry->links);
put_css_set(cg_entry->cg);
kfree(cg_entry);
out_put_css_set_refs:
if (retval) {
for (i = 0; i < group_size; i++) {
tc = flex_array_get(group, i);
if (!tc->cg)
break;
put_css_set(tc->cg);
}
}
out_cancel_attach:
/* same deal as in cgroup_attach_task */
if (retval) {
for_each_subsys(root, ss) {
if (ss == failed_ss)
Expand Down

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