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>

1 files changed, 0 insertions, 8 deletions

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index e38681f4912d..d0b380ee7d67 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),
@@ -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;