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

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Message-ID: <alpine.LFD.2.00.1304271342260.1633@ja.ssi.bg>
Date:	Sat, 27 Apr 2013 14:32:48 +0300 (EEST)
From:	Julian Anastasov <ja@....bg>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	paulmck@...ux.vnet.ibm.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


	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)
	cond_resched();
#endif
	rcu_read_lock();

Regards

--
Julian Anastasov <ja@....bg>
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