Re: [PATCH v2 1/2] sched: Add cond_resched_rcu_lock() helper

[Julian Anastasov <ja@ssi.bg>]

	Hello,

On Thu, 2 May 2013, Peter Zijlstra wrote:

> On Wed, May 01, 2013 at 09:22:08PM +0300, Julian Anastasov wrote:
> > > +extern int __cond_resched_rcu(void);
> > > +
> > > +#define cond_resched_rcu() ({			\
> > > +	__might_sleep(__FILE__, __LINE__, 0);	\
> > 
> > 	I see your goal. But digging into __might_sleep()
> > I see that rcu_sleep_check() will scream for the non-preempt
> > case because we are under rcu_read_lock.
> 
> 
> #ifdef CONFIG_PREEMPT_RCU
> #define PREEMPT_RCU_OFFSET 0
> #else
> #define PREEMPT_RCU_OFFSET 1
> #endif
> 
> #define cond_resched_rcu() ({	\
> 	__might_sleep(__FILE__, __LINE__, PREEMPT_RCU_OFFSET);	\
> 	__cond_resched_rcu();	\
> })
> 
> Should work I think..

	I implemented your idea.

	I tested the following patch in 2 variants,
TINY_RCU and CONFIG_TREE_PREEMPT_RCU. I see the
error if extra rcu_read_lock is added for testing.

	I'm using the PREEMPT_ACTIVE flag to indicate
that we are already under lock. It should work because
__might_sleep is not called with such bit. I also tried to
add new flag in include/linux/hardirq.h but PREEMPT_ACTIVE
depends on the arch, so this alternative looked difficult to
implement.

 include/linux/rcupdate.h |    7 ++++---
 include/linux/sched.h    |   14 ++++++++++++++
 kernel/sched/core.c      |   20 ++++++++++++++++++--
 3 files changed, 36 insertions(+), 5 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index b758ce1..b594759 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -480,9 +480,10 @@ static inline void rcu_preempt_sleep_check(void)
 }
 #endif /* #else #ifdef CONFIG_PROVE_RCU */
 
-#define rcu_sleep_check()						\
+#define rcu_sleep_check(locked)						\
 	do {								\
-		rcu_preempt_sleep_check();				\
+		if (!(locked))						\
+			rcu_preempt_sleep_check();			\
 		rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map),	\
 				   "Illegal context switch in RCU-bh"	\
 				   " read-side critical section");	\
@@ -494,7 +495,7 @@ static inline void rcu_preempt_sleep_check(void)
 #else /* #ifdef CONFIG_PROVE_RCU */
 
 #define rcu_lockdep_assert(c, s) do { } while (0)
-#define rcu_sleep_check() do { } while (0)
+#define rcu_sleep_check(locked) do { } while (0)
 
 #endif /* #else #ifdef CONFIG_PROVE_RCU */
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index e692a02..027deea 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2608,6 +2608,20 @@ extern int __cond_resched_softirq(void);
 	__cond_resched_softirq();					\
 })
 
+#ifdef CONFIG_PREEMPT_RCU
+#define PREEMPT_RCU_OFFSET	1
+#else
+#define PREEMPT_RCU_OFFSET	PREEMPT_CHECK_OFFSET
+#endif
+
+extern int __cond_resched_rcu(void);
+
+#define cond_resched_rcu() ({					\
+	__might_sleep(__FILE__, __LINE__, PREEMPT_ACTIVE |	\
+					  PREEMPT_RCU_OFFSET);	\
+	__cond_resched_rcu();					\
+})
+
 /*
  * Does a critical section need to be broken due to another
  * task waiting?: (technically does not depend on CONFIG_PREEMPT,
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 67d0465..2724be7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2793,7 +2793,7 @@ static inline void schedule_debug(struct task_struct *prev)
 	 */
 	if (unlikely(in_atomic_preempt_off() && !prev->exit_state))
 		__schedule_bug(prev);
-	rcu_sleep_check();
+	rcu_sleep_check(0);
 
 	profile_hit(SCHED_PROFILING, __builtin_return_address(0));
 
@@ -4364,6 +4364,20 @@ int __sched __cond_resched_softirq(void)
 }
 EXPORT_SYMBOL(__cond_resched_softirq);
 
+int __sched __cond_resched_rcu(void)
+{
+#ifndef CONFIG_PREEMPT_RCU
+	if (should_resched()) {
+		rcu_read_unlock();
+		__cond_resched();
+		rcu_read_lock();
+		return 1;
+	}
+#endif
+	return 0;
+}
+EXPORT_SYMBOL(__cond_resched_rcu);
+
 /**
  * yield - yield the current processor to other threads.
  *
@@ -7062,7 +7076,9 @@ void __might_sleep(const char *file, int line, int preempt_offset)
 {
 	static unsigned long prev_jiffy;	/* ratelimiting */
 
-	rcu_sleep_check(); /* WARN_ON_ONCE() by default, no rate limit reqd. */
+	/* WARN_ON_ONCE() by default, no rate limit reqd. */
+	rcu_sleep_check(preempt_offset & PREEMPT_ACTIVE);
+	preempt_offset &= ~PREEMPT_ACTIVE;
 	if ((preempt_count_equals(preempt_offset) && !irqs_disabled()) ||
 	    system_state != SYSTEM_RUNNING || oops_in_progress)
 		return;
-- 
1.7.3.4


Regards

--
Julian Anastasov <ja@ssi.bg>