locking, rcu, cgroup: Avoid synchronize_sched() in __cgroup_procs_write()
The current percpu-rwsem read side is entirely free of serializing insns at the cost of having a synchronize_sched() in the write path. The latency of the synchronize_sched() is too high for cgroups. The commit 1ed13287 talks about the write path being a fairly cold path but this is not the case for Android which moves task to the foreground cgroup and back around binder IPC calls from foreground processes to background processes, so it is significantly hotter than human initiated operations. Switch cgroup_threadgroup_rwsem into the slow mode for now to avoid the problem, hopefully it should not be that slow after another commit: 80127a39 ("locking/percpu-rwsem: Optimize readers and reduce global impact"). We could just add rcu_sync_enter() into cgroup_init() but we do not want another synchronize_sched() at boot time, so this patch adds the new helper which doesn't block but currently can only be called before the first use. Reported-by:John Stultz <john.stultz@linaro.org> Reported-by:
Dmitry Shmidt <dimitrysh@google.com> Signed-off-by:
Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by:
Oleg Nesterov <oleg@redhat.com> Signed-off-by:
Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Colin Cross <ccross@google.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Rom Lemarchand <romlem@google.com> Cc: Tejun Heo <tj@kernel.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Todd Kjos <tkjos@google.com> Link: http://lkml.kernel.org/r/20160811165413.GA22807@redhat.com Signed-off-by:
Ingo Molnar <mingo@kernel.org>
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