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Subject[PATCH] cgroup: remove extra calls to find_existing_css_set
Date
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>
Cc: 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>
---
kernel/cgroup.c | 142 +++++++++++--------------------------------------------
1 files changed, 28 insertions(+), 114 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1626152..70a797e 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -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 {
@@ -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
@@ -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);
@@ -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;
}

/**
@@ -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)
@@ -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)
@@ -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
@@ -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
@@ -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++;
@@ -2156,21 +2075,16 @@ 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;
}
}

@@ -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. */

@@ -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)
--
1.7.7.3


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