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

To: Eric Dumazet <eric.dumazet@xxxxxxxxx>, Julian Anastasov <ja@xxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Subject: [PATCH ipvs-next v3 1/2] sched: add cond_resched_rcu() helper
Cc: lvs-devel@xxxxxxxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, netfilter-devel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>, Dipankar Sarma <dipankar@xxxxxxxxxx>, Simon Horman <horms@xxxxxxxxxxxx>
From: Simon Horman <horms@xxxxxxxxxxxx>
Date: Wed, 22 May 2013 14:50:31 +0900
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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