[PATCH ipvs-next v3 1/2] sched: add cond_resched_rcu() helper

From: Simon Horman
Date: Wed May 22 2013 - 01:50:59 EST

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@xxxxxx>
Signed-off-by: Julian Anastasov <ja@xxxxxx>
Signed-off-by: Simon Horman <horms@xxxxxxxxxxxx>
include/linux/sched.h | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index e692a02..2080446 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2608,6 +2608,15 @@ extern int __cond_resched_softirq(void);
__cond_resched_softirq(); \

+static inline void cond_resched_rcu(void)
+ rcu_read_unlock();
+ cond_resched();
+ rcu_read_lock();
* Does a critical section need to be broken due to another
* task waiting?: (technically does not depend on CONFIG_PREEMPT,

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