Re: [PATCH 1/2] rcu: export rcu_note_context_switch() function

[Gleb Natapov <gleb@redhat.com>]

On Fri, Apr 29, 2011 at 01:39:04AM -0700, Paul E. McKenney wrote:
> On Fri, Apr 29, 2011 at 01:36:18AM -0700, Paul E. McKenney wrote:
> > On Thu, Apr 28, 2011 at 12:52:02PM +0300, Gleb Natapov wrote:
> > 
> > Hmmm....  This is interesting.  KVM being a module, we either expand
> > TINY_RCU's size a bit by making rcu_note_context_switch() be a real
> > function in rcutiny.c and adding an export, or we expand it by adding
> > two exports.
> > 
> > I would like to solve this without making TINY_RCU larger, and preferably
> > by making it smaller.  Any ideas come to mind?  (Other than making
> > KVM depend on CONFIG_SMP, which sounds too much like throwing out the
> > baby with the bathwater.)
> 
> Nothing quite like hitting "send" to make an idea show up...
> 
> In a UP kernel, does it actually help anything to have KVM
> tell RCU about executing in a guest?  If not, could we have a
> rcu_note_context_switch_kvm() that is a static inline empty function in
> TINY_RCU and maps to rcu_note_context_switch() for TREE_RCU?
> 
That will work, but does making rcu_note_context_switch() out of line
actually increase kernel size? The function is called in two places
currently, so by making it out of line we make two calling site smaller.
Will measure it next week.

> 							Thanx, Paul
> 
> > > Signed-off-by: Gleb Natapov <gleb@redhat.com>
> > > ---
> > >  include/linux/rcutiny.h |    6 +-----
> > >  kernel/rcutiny.c        |    7 +++++++
> > >  kernel/rcutree.c        |    1 +
> > >  3 files changed, 9 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> > > index 30ebd7c..8e5f7cf 100644
> > > --- a/include/linux/rcutiny.h
> > > +++ b/include/linux/rcutiny.h
> > > @@ -93,11 +93,7 @@ static inline int rcu_needs_cpu(int cpu)
> > > 
> > >  #endif /* #else #ifdef CONFIG_TINY_RCU */
> > > 
> > > -static inline void rcu_note_context_switch(int cpu)
> > > -{
> > > -	rcu_sched_qs(cpu);
> > > -	rcu_preempt_note_context_switch();
> > > -}
> > > +extern void rcu_note_context_switch(int cpu);
> > > 
> > >  /*
> > >   * Return the number of grace periods.
> > > diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
> > > index 0c343b9..3d715a4 100644
> > > --- a/kernel/rcutiny.c
> > > +++ b/kernel/rcutiny.c
> > > @@ -78,6 +78,13 @@ void rcu_exit_nohz(void)
> > > 
> > >  #endif /* #ifdef CONFIG_NO_HZ */
> > > 
> > > +void rcu_note_context_switch(int cpu)
> > > +{
> > > +	rcu_sched_qs(cpu);
> > > +	rcu_preempt_note_context_switch();
> > > +}
> > > +EXPORT_SYMBOL_GPL(rcu_note_context_switch);
> > > +
> > >  /*
> > >   * Helper function for rcu_qsctr_inc() and rcu_bh_qsctr_inc().
> > >   * Also disable irqs to avoid confusion due to interrupt handlers
> > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > > index dd4aea8..0837d63 100644
> > > --- a/kernel/rcutree.c
> > > +++ b/kernel/rcutree.c
> > > @@ -124,6 +124,7 @@ void rcu_note_context_switch(int cpu)
> > >  	rcu_sched_qs(cpu);
> > >  	rcu_preempt_note_context_switch(cpu);
> > >  }
> > > +EXPORT_SYMBOL_GPL(rcu_note_context_switch);
> > > 
> > >  #ifdef CONFIG_NO_HZ
> > >  DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
> > > -- 
> > > 1.7.2.3
> > > 
> > > --
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--
			Gleb.