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

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Message-ID: <20130429213002.GP3780@linux.vnet.ibm.com>
Date:	Mon, 29 Apr 2013 14:30:02 -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 Tue, Apr 30, 2013 at 12:08:18AM +0300, Julian Anastasov wrote:
> 
> 	Hello,
> 
> On Sat, 27 Apr 2013, Paul E. McKenney wrote:
> 
> > On Sat, Apr 27, 2013 at 02:32:48PM +0300, Julian Anastasov wrote:
> > > 
> > > 	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.
> 
> 	Hm, is this correct? If I follow the ifdefs
> preempt_schedule is called when CONFIG_PREEMPT is
> defined _and_ CONFIG_PREEMPT_RCU is not defined.
> Your example for CONFIG_PREEMPT_RCU is the opposite to this?

Yep, I really did intend to say "#ifndef CONFIG_PREEMPT_RCU".

A couple of things to keep in mind:

1.	Although rcu_read_unlock() does map to preempt_enable() for
	CONFIG_TINY_RCU and CONFIG_TREE_RCU, the current Kconfig refuses
	to allow either CONFIG_TINY_RCU or CONFIG_TREE_RCU to be selected
	if CONFIG_PREEMPT=y.

2.	In the CONFIG_PREEMPT_RCU case, __rcu_read_unlock() will check
	to see if the RCU core needs to be informed, so there is no
	need to invoke cond_resched() in that case.

3.	If we drop your "|| defined(CONFIG_PREEMPT_RCU)", we get an
	almost-synonym for my "#ifndef CONFIG_PREEMPT_RCU".  The "almost"
	applies to older kernels due to the possibility of having a
	CONFIG_TINY_PREEMPT_RCU kernel -- but this possibility is going
	away soon.

Make sense?

							Thanx, Paul

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

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