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 <ja@ssi.bg>
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Change-Id: I5f36f86484198f9064725d424c3d91d5fac8e1d4
Origin: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=f6f3c437d09e2f62533034e67bfb4385191e992c
Backported-by: Maciej Wereski <m.wereski@partner.samsung.com>
Signed-off-by: Maciej Wereski <m.wereski@partner.samsung.com>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index f87e9a8..e12df9e 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2462,6 +2462,15 @@ 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,