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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 <ja@xxxxxx>
>
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