sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem
Note: This commit was originally committed asd59cfc09c3
but got reverted by0c986253b9
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>
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@ -1149,10 +1149,6 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
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tty_audit_fork(sig);
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sched_autogroup_fork(sig);
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#ifdef CONFIG_CGROUPS
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init_rwsem(&sig->group_rwsem);
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#endif
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sig->oom_score_adj = current->signal->oom_score_adj;
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sig->oom_score_adj_min = current->signal->oom_score_adj_min;
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