sched: add cond_resched_rcu() helper
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 <84572ef2253ef81e2d8cd8c65849f4d9a3881f47@ssi.bg>
Signed-off-by: Julian Anastasov <84572ef2253ef81e2d8cd8c65849f4d9a3881f47@ssi.bg>
Signed-off-by: Simon Horman <a105fd58f578dea0be108941d4c59d15396e1855@verge.net.au>
Acked-by: Peter Zijlstra <3fddac958924aef220f202ca567388ddab3f14a8@infradead.org>
Signed-off-by: Pablo Neira Ayuso <707d14912bb250caf67dfe0ea4035681fbfc4f56@netfilter.org>

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