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

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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>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
This commit is contained in:
Tejun Heo 2015-05-13 16:35:17 -04:00
commit d59cfc09c3
6 changed files with 46 additions and 83 deletions
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27
include/linux/cgroup-defs.h
View file

@ -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 */