Re: [PATCH 2/2] ipvs: Use cond_resched_rcu_lock() helper whendumping connections
From: Paul E. McKenney
Date: Mon Apr 29 2013 - 17:30:44 EST
On Tue, Apr 30, 2013 at 12:08:18AM +0300, Julian Anastasov wrote:
> 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
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
> > > cond_resched();
> > > #endif
> > > rcu_read_lock();
> Julian Anastasov <ja@xxxxxx>
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