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Message-Id: <1431549318-16756-3-git-send-email-tj@kernel.org>
Date: Wed, 13 May 2015 16:35:17 -0400
From: Tejun Heo <tj@...nel.org>
To: lizefan@...wei.com
Cc: cgroups@...r.kernel.org, mingo@...hat.com, peterz@...radead.org,
linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>
Subject: [PATCH 2/3] sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem
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@...nel.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
---
include/linux/cgroup-defs.h | 27 ++++++++++++++--
include/linux/init_task.h | 8 -----
include/linux/sched.h | 12 -------
init/Kconfig | 1 +
kernel/cgroup.c | 77 ++++++++++++---------------------------------
kernel/fork.c | 4 ---
6 files changed, 46 insertions(+), 83 deletions(-)
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 1b8c938..7d83d7f 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -461,8 +461,31 @@ struct cgroup_subsys {
unsigned int depends_on;
};
-void cgroup_threadgroup_change_begin(struct task_struct *tsk);
-void cgroup_threadgroup_change_end(struct task_struct *tsk);
+extern struct percpu_rw_semaphore cgroup_threadgroup_rwsem;
+
+/**
+ * cgroup_threadgroup_change_begin - threadgroup exclusion for cgroups
+ * @tsk: target task
+ *
+ * Called from threadgroup_change_begin() and allows cgroup operations to
+ * synchronize against threadgroup changes using a percpu_rw_semaphore.
+ */
+static inline void cgroup_threadgroup_change_begin(struct task_struct *tsk)
+{
+ percpu_down_read(&cgroup_threadgroup_rwsem);
+}
+
+/**
+ * cgroup_threadgroup_change_end - threadgroup exclusion for cgroups
+ * @tsk: target task
+ *
+ * Called from threadgroup_change_end(). Counterpart of
+ * cgroup_threadcgroup_change_begin().
+ */
+static inline void cgroup_threadgroup_change_end(struct task_struct *tsk)
+{
+ percpu_up_read(&cgroup_threadgroup_rwsem);
+}
#else /* CONFIG_CGROUPS */
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 696d223..0cc0bbf 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -25,13 +25,6 @@
extern struct files_struct init_files;
extern struct fs_struct init_fs;
-#ifdef CONFIG_CGROUPS
-#define INIT_GROUP_RWSEM(sig) \
- .group_rwsem = __RWSEM_INITIALIZER(sig.group_rwsem),
-#else
-#define INIT_GROUP_RWSEM(sig)
-#endif
-
#ifdef CONFIG_CPUSETS
#define INIT_CPUSET_SEQ(tsk) \
.mems_allowed_seq = SEQCNT_ZERO(tsk.mems_allowed_seq),
@@ -56,7 +49,6 @@ extern struct fs_struct init_fs;
}, \
.cred_guard_mutex = \
__MUTEX_INITIALIZER(sig.cred_guard_mutex), \
- INIT_GROUP_RWSEM(sig) \
}
extern struct nsproxy init_nsproxy;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 5ee2900..add524a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -743,18 +743,6 @@ struct signal_struct {
unsigned audit_tty_log_passwd;
struct tty_audit_buf *tty_audit_buf;
#endif
-#ifdef CONFIG_CGROUPS
- /*
- * group_rwsem prevents new tasks from entering the threadgroup and
- * member tasks from exiting,a more specifically, setting of
- * PF_EXITING. fork and exit paths are protected with this rwsem
- * using threadgroup_change_begin/end(). Users which require
- * threadgroup to remain stable should use threadgroup_[un]lock()
- * which also takes care of exec path. Currently, cgroup is the
- * only user.
- */
- struct rw_semaphore group_rwsem;
-#endif
oom_flags_t oom_flags;
short oom_score_adj; /* OOM kill score adjustment */
diff --git a/init/Kconfig b/init/Kconfig
index dc24dec..b9b824b 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -938,6 +938,7 @@ config NUMA_BALANCING_DEFAULT_ENABLED
menuconfig CGROUPS
bool "Control Group support"
select KERNFS
+ select PERCPU_RWSEM
help
This option adds support for grouping sets of processes together, for
use with process control subsystems such as Cpusets, CFS, memory
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 9309452..5435b9d 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_assert(rcu_read_lock_held() || \
lockdep_is_held(&cgroup_mutex), \
@@ -848,48 +851,6 @@ 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;
@@ -2094,9 +2055,9 @@ 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);
@@ -2153,10 +2114,11 @@ 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,
@@ -2259,7 +2221,7 @@ 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().
@@ -2387,7 +2349,7 @@ 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)
@@ -2480,7 +2442,7 @@ 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)) {
/*
@@ -2490,7 +2452,7 @@ 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;
}
@@ -2498,7 +2460,7 @@ retry_find_task:
ret = cgroup_attach_task(cgrp, tsk, threadgroup);
- threadgroup_unlock(tsk);
+ percpu_up_write(&cgroup_threadgroup_rwsem);
put_task_struct(tsk);
out_unlock_cgroup:
@@ -2703,17 +2665,17 @@ 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))
@@ -5031,6 +4993,7 @@ 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));
diff --git a/kernel/fork.c b/kernel/fork.c
index 03c1eaa..9531275 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1144,10 +1144,6 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
tty_audit_fork(sig);
sched_autogroup_fork(sig);
-#ifdef CONFIG_CGROUPS
- init_rwsem(&sig->group_rwsem);
-#endif
-
sig->oom_score_adj = current->signal->oom_score_adj;
sig->oom_score_adj_min = current->signal->oom_score_adj_min;
--
2.1.0
--
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