netdev - Re: [PATCH 2/2] ipvs: Use cond_resched_rcu_lock() helper when dumping connections

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Message-ID: <20130427162049.GB3780@linux.vnet.ibm.com>
Date:	Sat, 27 Apr 2013 09:20:49 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Julian Anastasov <ja@....bg>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Simon Horman <horms@...ge.net.au>,
	Ingo Molnar <mingo@...hat.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>, dhaval.giani@...il.com
Subject: Re: [PATCH 2/2] ipvs: Use cond_resched_rcu_lock() helper when
 dumping connections

On Sat, Apr 27, 2013 at 02:32:48PM +0300, Julian Anastasov wrote:
> 
> 	Hello,
> 
> On Fri, 26 Apr 2013, Eric Dumazet wrote:
> 
> > On Fri, 2013-04-26 at 10:48 -0700, Paul E. McKenney wrote:
> > 
> > > Don't get me wrong, I am not opposing cond_resched_rcu_lock() because it
> > > will be difficult to validate.  For one thing, until there are a lot of
> > > them, manual inspection is quite possible.  So feel free to apply my
> > > Acked-by to the patch.
> > 
> > One question : If some thread(s) is(are) calling rcu_barrier() and
> > waiting we exit from rcu_read_lock() section, is need_resched() enough
> > for allowing to break the section ?
> > 
> > If not, maybe we should not test need_resched() at all.
> > 
> > rcu_read_unlock();
> > cond_resched();
> > rcu_read_lock();
> 
> 	So, I assume, to help realtime kernels and rcu_barrier
> it is not a good idea to guard rcu_read_unlock with checks.
> I see that rcu_read_unlock will try to reschedule in the 
> !CONFIG_PREEMPT_RCU case (via preempt_enable), can we
> use ifdefs to avoid double TIF_NEED_RESCHED check?:
> 
> 	rcu_read_unlock();
> #if !defined(CONFIG_PREEMPT) || defined(CONFIG_PREEMPT_RCU)

I would instead suggest something like:

#ifndef CONFIG_PREEMPT_RCU

But yes, in the CONFIG_PREEMPT_RCU case, the cond_resched() is not
needed.

							Thanx, Paul

> 	cond_resched();
> #endif
> 	rcu_read_lock();
> 
> Regards
> 
> --
> Julian Anastasov <ja@....bg>
> 

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