sched: add cond_resched_rcu() helper
This is intended for use in loops which read data protected by RCU and may have a large number of iterations. Such an example is dumping the list of connections known to IPVS: ip_vs_conn_array() and ip_vs_conn_seq_next(). The benefits are for CONFIG_PREEMPT_RCU=y where we save CPU cycles by moving rcu_read_lock and rcu_read_unlock out of large loops but still allowing the current task to be preempted after every loop iteration for the CONFIG_PREEMPT_RCU=n case. The call to cond_resched() is not needed when CONFIG_PREEMPT_RCU=y. Thanks to Paul E. McKenney for explaining this and for the final version that checks the context with CONFIG_DEBUG_ATOMIC_SLEEP=y for all possible configurations. The function can be empty in the CONFIG_PREEMPT_RCU case, rcu_read_lock and rcu_read_unlock are not needed in this case because the task can be preempted on indication from scheduler. Thanks to Peter Zijlstra for catching this and for his help in trying a solution that changes __might_sleep. Initial cond_resched_rcu_lock() function suggested by Eric Dumazet. Tested-by: Julian Anastasov <ja@ssi.bg> Signed-off-by: Julian Anastasov <ja@ssi.bg> Signed-off-by: Simon Horman <horms@verge.net.au> Acked-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
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@ -2444,6 +2444,15 @@ extern int __cond_resched_softirq(void);
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__cond_resched_softirq(); \
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})
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static inline void cond_resched_rcu(void)
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{
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#if defined(CONFIG_DEBUG_ATOMIC_SLEEP) || !defined(CONFIG_PREEMPT_RCU)
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rcu_read_unlock();
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cond_resched();
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rcu_read_lock();
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#endif
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}
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/*
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* Does a critical section need to be broken due to another
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* task waiting?: (technically does not depend on CONFIG_PREEMPT,
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