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authorTejun Heo <tj@kernel.org>2015-09-16 12:53:17 -0400
committerTejun Heo <tj@kernel.org>2015-09-16 12:53:17 -0400
commit1ed1328792ff46e4bb86a3d7f7be2971f4549f6c (patch)
tree53719cfc0bf81bc7e6fb522944553d9b4fa36cbf /kernel/cgroup.c
parent0c986253b939cc14c69d4adbe2b4121bdf4aa220 (diff)
sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem
Note: This commit was originally committed as d59cfc09c32a but got reverted by 0c986253b939 due to the performance regression from the percpu_rwsem write down/up operations added to cgroup task migration path. percpu_rwsem changes which alleviate the performance issue are pending for v4.4-rc1 merge window. Re-apply. The cgroup side of threadgroup locking uses signal_struct->group_rwsem to synchronize against threadgroup changes. This per-process rwsem adds small overhead to thread creation, exit and exec paths, forces cgroup code paths to do lock-verify-unlock-retry dance in a couple places and makes it impossible to atomically perform operations across multiple processes. This patch replaces signal_struct->group_rwsem with a global percpu_rwsem cgroup_threadgroup_rwsem which is cheaper on the reader side and contained in cgroups proper. This patch converts one-to-one. This does make writer side heavier and lower the granularity; however, cgroup process migration is a fairly cold path, we do want to optimize thread operations over it and cgroup migration operations don't take enough time for the lower granularity to matter. Signed-off-by: Tejun Heo <tj@kernel.org> Link: http://lkml.kernel.org/g/55F8097A.7000206@de.ibm.com Cc: Ingo Molnar <mingo@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org>
Diffstat (limited to 'kernel/cgroup.c')
-rw-r--r--kernel/cgroup.c77
1 files changed, 20 insertions, 57 deletions
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 2c9eae6ad970..115091efa889 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -46,6 +46,7 @@
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/rwsem.h>
+#include <linux/percpu-rwsem.h>
#include <linux/string.h>
#include <linux/sort.h>
#include <linux/kmod.h>
@@ -103,6 +104,8 @@ static DEFINE_SPINLOCK(cgroup_idr_lock);
*/
static DEFINE_SPINLOCK(release_agent_path_lock);
+struct percpu_rw_semaphore cgroup_threadgroup_rwsem;
+
#define cgroup_assert_mutex_or_rcu_locked() \
RCU_LOCKDEP_WARN(!rcu_read_lock_held() && \
!lockdep_is_held(&cgroup_mutex), \
@@ -871,48 +874,6 @@ static struct css_set *find_css_set(struct css_set *old_cset,
return cset;
}
-void cgroup_threadgroup_change_begin(struct task_struct *tsk)
-{
- down_read(&tsk->signal->group_rwsem);
-}
-
-void cgroup_threadgroup_change_end(struct task_struct *tsk)
-{
- up_read(&tsk->signal->group_rwsem);
-}
-
-/**
- * threadgroup_lock - lock threadgroup
- * @tsk: member task of the threadgroup to lock
- *
- * Lock the threadgroup @tsk belongs to. No new task is allowed to enter
- * and member tasks aren't allowed to exit (as indicated by PF_EXITING) or
- * change ->group_leader/pid. This is useful for cases where the threadgroup
- * needs to stay stable across blockable operations.
- *
- * fork and exit explicitly call threadgroup_change_{begin|end}() for
- * synchronization. While held, no new task will be added to threadgroup
- * and no existing live task will have its PF_EXITING set.
- *
- * de_thread() does threadgroup_change_{begin|end}() when a non-leader
- * sub-thread becomes a new leader.
- */
-static void threadgroup_lock(struct task_struct *tsk)
-{
- down_write(&tsk->signal->group_rwsem);
-}
-
-/**
- * threadgroup_unlock - unlock threadgroup
- * @tsk: member task of the threadgroup to unlock
- *
- * Reverse threadgroup_lock().
- */
-static inline void threadgroup_unlock(struct task_struct *tsk)
-{
- up_write(&tsk->signal->group_rwsem);
-}
-
static struct cgroup_root *cgroup_root_from_kf(struct kernfs_root *kf_root)
{
struct cgroup *root_cgrp = kf_root->kn->priv;
@@ -2113,9 +2074,9 @@ static void cgroup_task_migrate(struct cgroup *old_cgrp,
lockdep_assert_held(&css_set_rwsem);
/*
- * We are synchronized through threadgroup_lock() against PF_EXITING
- * setting such that we can't race against cgroup_exit() changing the
- * css_set to init_css_set and dropping the old one.
+ * We are synchronized through cgroup_threadgroup_rwsem against
+ * PF_EXITING setting such that we can't race against cgroup_exit()
+ * changing the css_set to init_css_set and dropping the old one.
*/
WARN_ON_ONCE(tsk->flags & PF_EXITING);
old_cset = task_css_set(tsk);
@@ -2172,10 +2133,11 @@ static void cgroup_migrate_finish(struct list_head *preloaded_csets)
* @src_cset and add it to @preloaded_csets, which should later be cleaned
* up by cgroup_migrate_finish().
*
- * This function may be called without holding threadgroup_lock even if the
- * target is a process. Threads may be created and destroyed but as long
- * as cgroup_mutex is not dropped, no new css_set can be put into play and
- * the preloaded css_sets are guaranteed to cover all migrations.
+ * This function may be called without holding cgroup_threadgroup_rwsem
+ * even if the target is a process. Threads may be created and destroyed
+ * but as long as cgroup_mutex is not dropped, no new css_set can be put
+ * into play and the preloaded css_sets are guaranteed to cover all
+ * migrations.
*/
static void cgroup_migrate_add_src(struct css_set *src_cset,
struct cgroup *dst_cgrp,
@@ -2278,7 +2240,7 @@ err:
* @threadgroup: whether @leader points to the whole process or a single task
*
* Migrate a process or task denoted by @leader to @cgrp. If migrating a
- * process, the caller must be holding threadgroup_lock of @leader. The
+ * process, the caller must be holding cgroup_threadgroup_rwsem. The
* caller is also responsible for invoking cgroup_migrate_add_src() and
* cgroup_migrate_prepare_dst() on the targets before invoking this
* function and following up with cgroup_migrate_finish().
@@ -2406,7 +2368,7 @@ out_release_tset:
* @leader: the task or the leader of the threadgroup to be attached
* @threadgroup: attach the whole threadgroup?
*
- * Call holding cgroup_mutex and threadgroup_lock of @leader.
+ * Call holding cgroup_mutex and cgroup_threadgroup_rwsem.
*/
static int cgroup_attach_task(struct cgroup *dst_cgrp,
struct task_struct *leader, bool threadgroup)
@@ -2528,7 +2490,7 @@ retry_find_task:
get_task_struct(tsk);
rcu_read_unlock();
- threadgroup_lock(tsk);
+ percpu_down_write(&cgroup_threadgroup_rwsem);
if (threadgroup) {
if (!thread_group_leader(tsk)) {
/*
@@ -2538,7 +2500,7 @@ retry_find_task:
* try again; this is
* "double-double-toil-and-trouble-check locking".
*/
- threadgroup_unlock(tsk);
+ percpu_up_write(&cgroup_threadgroup_rwsem);
put_task_struct(tsk);
goto retry_find_task;
}
@@ -2548,7 +2510,7 @@ retry_find_task:
if (!ret)
ret = cgroup_attach_task(cgrp, tsk, threadgroup);
- threadgroup_unlock(tsk);
+ percpu_up_write(&cgroup_threadgroup_rwsem);
put_task_struct(tsk);
out_unlock_cgroup:
@@ -2751,17 +2713,17 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
goto out_finish;
last_task = task;
- threadgroup_lock(task);
+ percpu_down_write(&cgroup_threadgroup_rwsem);
/* raced against de_thread() from another thread? */
if (!thread_group_leader(task)) {
- threadgroup_unlock(task);
+ percpu_up_write(&cgroup_threadgroup_rwsem);
put_task_struct(task);
continue;
}
ret = cgroup_migrate(src_cset->dfl_cgrp, task, true);
- threadgroup_unlock(task);
+ percpu_up_write(&cgroup_threadgroup_rwsem);
put_task_struct(task);
if (WARN(ret, "cgroup: failed to update controllers for the default hierarchy (%d), further operations may crash or hang\n", ret))
@@ -5083,6 +5045,7 @@ int __init cgroup_init(void)
unsigned long key;
int ssid, err;
+ BUG_ON(percpu_init_rwsem(&cgroup_threadgroup_rwsem));
BUG_ON(cgroup_init_cftypes(NULL, cgroup_dfl_base_files));
BUG_ON(cgroup_init_cftypes(NULL, cgroup_legacy_base_files));