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

[Julian Anastasov <ja@xxxxxx>]

        Hello,

On Wed, 1 May 2013, Peter Zijlstra wrote:

> On Tue, Apr 30, 2013 at 10:52:38AM +0300, Julian Anastasov wrote:
> > 
> >     Hello,
> > 
> > On Tue, 30 Apr 2013, Simon Horman wrote:
> > 
> > > Thanks, to clarify, just this:
> > > 
> > > static void inline cond_resched_rcu_lock(void)
> > > {
> > >   rcu_read_unlock();
> > > #ifndef CONFIG_PREEMPT_RCU
> > >   cond_resched();
> > > #endif
> > >   rcu_read_lock();
> > > }
> > 
> >     Yes, thanks!
> 
> OK, now I'm confused.. PREEMPT_RCU would preempt in any case, so why bother
> dropping rcu_read_lock() at all?
> 
> That is; the thing that makes sense to me is:
> 
> static void inline cond_resched_rcu_lock(void)
> {
> #ifdef CONFIG_PREEMPT_RCU

        You mean '#ifndef' here, right? But in the non-preempt
case is using the need_resched() needed? rcu_read_unlock
and rcu_read_lock do not generate code.

>       if (need_resched()) {
>               rcu_read_unlock();
>               cond_resched();
>               rcu_read_lock();
>       }
> #endif /* CONFIG_PREEMPT_RCU */
> }
> 
> That would have an rcu_read_lock() break and voluntary preemption point for
> non-preemptible RCU and not bother with the stuff for preemptible RCU.

        I see. So, can we choose one of both variants:

1. Your variant but with ifndef:

static void inline cond_resched_rcu_lock(void)
{
#ifndef CONFIG_PREEMPT_RCU
        if (need_resched()) {
                rcu_read_unlock();
                cond_resched();
                rcu_read_lock();
        }
#endif
}

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
}

Regards

--
Julian Anastasov <ja@xxxxxx>
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