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

From: "Paul E. McKenney" <[email-masked]>
Date: Wed, 1 May 2013 12:04:12 -0700

On Wed, May 01, 2013 at 09:22:08PM +0300, Julian Anastasov wrote:
> 
> 	Hello,
> 
> On Wed, 1 May 2013, Peter Zijlstra wrote:
> 
> > On Wed, May 01, 2013 at 05:22:05PM +0300, Julian Anastasov wrote:
> > 
> > > 2. Same without need_resched because cond_resched already
> > > performs the same checks:
> > > 
> > > static void inline cond_resched_rcu_lock(void)
> > > {
> > > #ifndef CONFIG_PREEMPT_RCU
> > > 	rcu_read_unlock();
> > > 	cond_resched();
> > > 	rcu_read_lock();
> > > #endif
> > > }
> > 
> > Ah so the 'problem' with this last version is that it does an unconditional /
> > unnessecary rcu_read_unlock().
> 
> 	It is just a barrier() for the non-preempt case.
> 
> > The below would be in line with all the other cond_resched*() implementations.
> 
> ...
> 
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 802a751..fd2c77f 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -2449,6 +2449,13 @@ extern int __cond_resched_softirq(void);
> >  	__cond_resched_softirq();					\
> >  })
> >  
> > +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.
> 
> 	What about such inline version:
> 
> static void inline cond_resched_rcu(void)
> {
> #ifndef CONFIG_PREEMPT_RCU
> 	rcu_read_unlock();
> 	__might_sleep(__FILE__, __LINE__, 0);
> 	cond_resched();
> 	rcu_read_lock();
> #else
> 	__might_sleep(__FILE__, __LINE__, 0);
> 	rcu_lockdep_assert(rcu_preempt_depth() == 1,
> 		"Illegal cond_resched_rcu() context");

The above requires that include/linux/sched.h be included.  This might
be OK, but please check the current intended uses.

						Thanx, Paul

> #endif
> }
> 
> 	It will restrict to single RCU lock level for all
> RCU implementations. But we don't have _cond_resched_rcu
> helper for two reasons:
> 
> - __might_sleep uses __FILE__, __LINE__
> - only cond_resched generates code, so need_resched() is
> avoided
> 
> > +	__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 7d7901a..2b3b4e6 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4358,6 +4358,20 @@ 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);
> > +
> 
> Regards
> 
> --
> Julian Anastasov <[email-masked]>
>