netdev - Re: [PATCH v2 1/2] sched: Add cond_resched_rcu

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Message-ID: <20130501091012.GB28253@dyad.programming.kicks-ass.net>
Date:	Wed, 1 May 2013 11:10:12 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Julian Anastasov <ja@....bg>
Cc:	Simon Horman <horms@...ge.net.au>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	lvs-devel@...r.kernel.org, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [PATCH v2 1/2] sched: Add cond_resched_rcu_lock() helper

On Tue, Apr 30, 2013 at 10:52:38AM +0300, Julian Anastasov wrote:
> 
> 	Hello,
> 
> On Tue, 30 Apr 2013, Simon Horman wrote:
> 
> > > > +static void inline cond_resched_rcu_lock(void)
> > > > +{
> > > > +	if (need_resched()) {
> > > 
> > > 	Ops, it should be without above need_resched.
> > 
> > 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
	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.
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