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Re: [PATCH 2/2] ipvs: Use cond_resched_rcu_lock() helper when dumping co

To: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Subject: Re: [PATCH 2/2] ipvs: Use cond_resched_rcu_lock() helper when dumping connections
Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Simon Horman <horms@xxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, lvs-devel@xxxxxxxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, netfilter-devel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>, Dipankar Sarma <dipankar@xxxxxxxxxx>, dhaval.giani@xxxxxxxxx
From: Julian Anastasov <ja@xxxxxx>
Date: Tue, 30 Apr 2013 02:12:10 +0300 (EEST)
        Hello,

On Mon, 29 Apr 2013, Paul E. McKenney wrote:

> On Tue, Apr 30, 2013 at 12:08:18AM +0300, Julian Anastasov wrote:
> > 
> >     Hello,
> > 
> > On Sat, 27 Apr 2013, Paul E. McKenney wrote:
> > 
> > > 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.

        I see, CONFIG_PREEMPT_RCU depends on CONFIG_PREEMPT

> 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.

        OK

> 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?

        Yes, thanks for the explanation!

        Simon, so lets do it as suggested by Eric and Paul:

        rcu_read_unlock();
#ifndef CONFIG_PREEMPT_RCU
        cond_resched();
#endif
        rcu_read_lock();

Regards

--
Julian Anastasov <ja@xxxxxx>
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