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

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Submitter Simon Horman
Date May 22, 2013, 5:50 a.m.
Message ID <1369201832-17163-2-git-send-email-horms@verge.net.au>
Download mbox | patch
Permalink /patch/245532/
State Accepted
Headers show

Comments

Simon Horman - May 22, 2013, 5:50 a.m.
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>
---
 include/linux/sched.h | 9 +++++++++
 1 file changed, 9 insertions(+)
Pablo Neira - May 23, 2013, 11:30 a.m.
On Wed, May 22, 2013 at 02:50:31PM +0900, Simon Horman wrote:
> 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.

Applied, thanks.
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Patch

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)
+{
+#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,