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3942a9bd7b
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 commit1ed1328792talks 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:80127a3968("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>
88 lines
2.6 KiB
C
88 lines
2.6 KiB
C
/*
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* RCU-based infrastructure for lightweight reader-writer locking
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, you can access it online at
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* http://www.gnu.org/licenses/gpl-2.0.html.
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*
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* Copyright (c) 2015, Red Hat, Inc.
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*
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* Author: Oleg Nesterov <oleg@redhat.com>
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*/
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#ifndef _LINUX_RCU_SYNC_H_
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#define _LINUX_RCU_SYNC_H_
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#include <linux/wait.h>
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#include <linux/rcupdate.h>
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enum rcu_sync_type { RCU_SYNC, RCU_SCHED_SYNC, RCU_BH_SYNC };
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/* Structure to mediate between updaters and fastpath-using readers. */
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struct rcu_sync {
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int gp_state;
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int gp_count;
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wait_queue_head_t gp_wait;
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int cb_state;
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struct rcu_head cb_head;
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enum rcu_sync_type gp_type;
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};
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extern void rcu_sync_lockdep_assert(struct rcu_sync *);
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/**
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* rcu_sync_is_idle() - Are readers permitted to use their fastpaths?
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* @rsp: Pointer to rcu_sync structure to use for synchronization
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*
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* Returns true if readers are permitted to use their fastpaths.
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* Must be invoked within an RCU read-side critical section whose
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* flavor matches that of the rcu_sync struture.
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*/
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static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
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{
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#ifdef CONFIG_PROVE_RCU
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rcu_sync_lockdep_assert(rsp);
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#endif
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return !rsp->gp_state; /* GP_IDLE */
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}
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extern void rcu_sync_init(struct rcu_sync *, enum rcu_sync_type);
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extern void rcu_sync_enter_start(struct rcu_sync *);
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extern void rcu_sync_enter(struct rcu_sync *);
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extern void rcu_sync_exit(struct rcu_sync *);
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extern void rcu_sync_dtor(struct rcu_sync *);
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#define __RCU_SYNC_INITIALIZER(name, type) { \
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.gp_state = 0, \
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.gp_count = 0, \
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.gp_wait = __WAIT_QUEUE_HEAD_INITIALIZER(name.gp_wait), \
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.cb_state = 0, \
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.gp_type = type, \
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}
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#define __DEFINE_RCU_SYNC(name, type) \
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struct rcu_sync_struct name = __RCU_SYNC_INITIALIZER(name, type)
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#define DEFINE_RCU_SYNC(name) \
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__DEFINE_RCU_SYNC(name, RCU_SYNC)
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#define DEFINE_RCU_SCHED_SYNC(name) \
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__DEFINE_RCU_SYNC(name, RCU_SCHED_SYNC)
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#define DEFINE_RCU_BH_SYNC(name) \
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__DEFINE_RCU_SYNC(name, RCU_BH_SYNC)
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#endif /* _LINUX_RCU_SYNC_H_ */
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