25502a6c13
The addition of preemptible RCU to treercu resulted in a bit of confusion and inefficiency surrounding the handling of context switches for RCU-sched and for RCU-preempt. For RCU-sched, a context switch is a quiescent state, pure and simple, just like it always has been. For RCU-preempt, a context switch is in no way a quiescent state, but special handling is required when a task blocks in an RCU read-side critical section. However, the callout from the scheduler and the outer loop in ksoftirqd still calls something named rcu_sched_qs(), whose name is no longer accurate. Furthermore, when rcu_check_callbacks() notes an RCU-sched quiescent state, it ends up unnecessarily (though harmlessly, aside from the performance hit) enqueuing the current task if it happens to be running in an RCU-preempt read-side critical section. This not only increases the maximum latency of scheduler_tick(), it also needlessly increases the overhead of the next outermost rcu_read_unlock() invocation. This patch addresses this situation by separating the notion of RCU's context-switch handling from that of RCU-sched's quiescent states. The context-switch handling is covered by rcu_note_context_switch() in general and by rcu_preempt_note_context_switch() for preemptible RCU. This permits rcu_sched_qs() to handle quiescent states and only quiescent states. It also reduces the maximum latency of scheduler_tick(), though probably by much less than a microsecond. Finally, it means that tasks within preemptible-RCU read-side critical sections avoid incurring the overhead of queuing unless there really is a context switch. Suggested-by: Lai Jiangshan <laijs@cn.fujitsu.com> Acked-by: Lai Jiangshan <laijs@cn.fujitsu.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Peter Zijlstra <peterz@infradead.org>
132 lines
2.7 KiB
C
132 lines
2.7 KiB
C
/*
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* Read-Copy Update mechanism for mutual exclusion, the Bloatwatch edition.
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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, write to the Free Software
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* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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*
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* Copyright IBM Corporation, 2008
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*
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* Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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*
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* For detailed explanation of Read-Copy Update mechanism see -
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* Documentation/RCU
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*/
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#ifndef __LINUX_TINY_H
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#define __LINUX_TINY_H
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#include <linux/cache.h>
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void rcu_sched_qs(int cpu);
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void rcu_bh_qs(int cpu);
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static inline void rcu_note_context_switch(int cpu)
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{
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rcu_sched_qs(cpu);
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}
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#define __rcu_read_lock() preempt_disable()
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#define __rcu_read_unlock() preempt_enable()
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#define __rcu_read_lock_bh() local_bh_disable()
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#define __rcu_read_unlock_bh() local_bh_enable()
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#define call_rcu_sched call_rcu
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#define rcu_init_sched() do { } while (0)
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extern void rcu_check_callbacks(int cpu, int user);
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static inline int rcu_needs_cpu(int cpu)
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{
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return 0;
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}
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/*
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* Return the number of grace periods.
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*/
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static inline long rcu_batches_completed(void)
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{
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return 0;
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}
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/*
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* Return the number of bottom-half grace periods.
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*/
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static inline long rcu_batches_completed_bh(void)
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{
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return 0;
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}
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extern int rcu_expedited_torture_stats(char *page);
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static inline void rcu_force_quiescent_state(void)
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{
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}
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static inline void rcu_bh_force_quiescent_state(void)
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{
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}
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static inline void rcu_sched_force_quiescent_state(void)
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{
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}
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extern void synchronize_sched(void);
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static inline void synchronize_rcu(void)
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{
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synchronize_sched();
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}
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static inline void synchronize_rcu_bh(void)
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{
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synchronize_sched();
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}
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static inline void synchronize_rcu_expedited(void)
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{
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synchronize_sched();
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}
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static inline void synchronize_rcu_bh_expedited(void)
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{
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synchronize_sched();
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}
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struct notifier_block;
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#ifdef CONFIG_NO_HZ
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extern void rcu_enter_nohz(void);
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extern void rcu_exit_nohz(void);
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#else /* #ifdef CONFIG_NO_HZ */
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static inline void rcu_enter_nohz(void)
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{
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}
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static inline void rcu_exit_nohz(void)
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{
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}
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#endif /* #else #ifdef CONFIG_NO_HZ */
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static inline void exit_rcu(void)
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{
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}
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static inline int rcu_preempt_depth(void)
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{
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return 0;
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}
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#endif /* __LINUX_RCUTINY_H */
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