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

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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
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12
include/linux/sched.h
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@@ -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 */