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: NJulian Anastasov <ja@ssi.bg> Signed-off-by: NJulian Anastasov <ja@ssi.bg> Signed-off-by: NSimon Horman <horms@verge.net.au> Acked-by: NPeter Zijlstra <peterz@infradead.org> Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>
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