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

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>


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