[Devel] [PATCH RHEL7 COMMIT] ms/sched: add cond_resched_rcu() helper

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-229.7.2.vz7.8.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-229.7.2.vz7.8.6
------>
commit f5375ae5711c334bb1305639dc08a45898a32f19
Author: Andrew Vagin <avagin at openvz.org>
Date:   Thu Oct 15 15:04:15 2015 +0400

    ms/sched: add cond_resched_rcu() helper
    
    The patch is required for CRIU.
    
    https://jira.sw.ru/browse/PSBM-39834
    
    From: Simon Horman <horms at verge.net.au>
    
    ML: f6f3c437d09e2f62533034e67bfb4385191e992c
    
    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 at ssi.bg>
    Signed-off-by: Julian Anastasov <ja at ssi.bg>
    Signed-off-by: Simon Horman <horms at verge.net.au>
    Acked-by: Peter Zijlstra <peterz at infradead.org>
    Signed-off-by: Pablo Neira Ayuso <pablo at netfilter.org>
    Signed-off-by: Andrew Vagin <avagin at openvz.org>
---
 include/linux/sched.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index e1bcabe..4560071 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2717,6 +2717,15 @@ extern int __cond_resched_softirq(void);
 	__cond_resched_softirq();					\
 })
 
+static inline void cond_resched_rcu(void)
+{
+#if defined(CONFIG_DEBUG_ATOMIC_SLEEP) || !defined(CONFIG_PREEMPT_RCU)
+	rcu_read_unlock();
+	cond_resched();
+	rcu_read_lock();
+#endif
+}
+
 /*
  * Does a critical section need to be broken due to another
  * task waiting?: (technically does not depend on CONFIG_PREEMPT,