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sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem 

Note: This commit was originally committed as d59cfc09c3 but got
      reverted by 0c986253b9 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>
This commit is contained in:
Tejun Heo 2015-09-16 12:53:17 -04:00
parent 0c986253b9
commit 1ed1328792
5 changed files with 45 additions and 83 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

27
include/linux/cgroup-defs.h
View File

@ -473,8 +473,31 @@ 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 */

8
include/linux/init_task.h
View File

@ -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),
@ -64,7 +57,6 @@ extern struct fs_struct init_fs;
INIT_PREV_CPUTIME(sig) \
.cred_guard_mutex = \
__MUTEX_INITIALIZER(sig.cred_guard_mutex), \
INIT_GROUP_RWSEM(sig) \
}
extern struct nsproxy init_nsproxy;

12
include/linux/sched.h
View File

@ -762,18 +762,6 @@ 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 */

77
kernel/cgroup.c
View File

@ -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 @@ static int cgroup_migrate_prepare_dst(struct cgroup *dst_cgrp,
* @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 @@ static int cgroup_migrate(struct cgroup *cgrp, struct task_struct *leader,
* @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 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
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 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
* 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 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
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));

4
kernel/fork.c
View File

@ -1149,10 +1149,6 @@ 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;