Re: [PATCH tip/core/rcu 2/2] rcu: Don't offload callbacks unless specifically requested

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Jul 11, 2014 at 03:47:37PM +0200, Frederic Weisbecker wrote:
> On Fri, Jul 11, 2014 at 06:35:03AM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > 
> > Enabling NO_HZ_FULL currently has the side effect of enabling callback
> > offloading on all CPUs.  This results in lots of additional rcuo kthreads,
> > and can also increase context switching and wakeups, even in cases where
> > callback offloading is neither needed nor particularly desirable.  This
> > commit therefore enables callback offloading on a given CPU only if
> > specifically requested at build time or boot time, or if that CPU has
> > been specifically designated (again, either at build time or boot time)
> > as a nohz_full CPU.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> ACK.

I thought you might like this one.  ;-)

							Thanx, Paul

> > ---
> >  init/Kconfig             | 4 ++--
> >  kernel/rcu/tree_plugin.h | 3 +++
> >  2 files changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/init/Kconfig b/init/Kconfig
> > index 9d76b99af1b9..9332d33346ac 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -737,7 +737,7 @@ choice
> >  
> >  config RCU_NOCB_CPU_NONE
> >  	bool "No build_forced no-CBs CPUs"
> > -	depends on RCU_NOCB_CPU && !NO_HZ_FULL
> > +	depends on RCU_NOCB_CPU && !NO_HZ_FULL_ALL
> >  	help
> >  	  This option does not force any of the CPUs to be no-CBs CPUs.
> >  	  Only CPUs designated by the rcu_nocbs= boot parameter will be
> > @@ -751,7 +751,7 @@ config RCU_NOCB_CPU_NONE
> >  
> >  config RCU_NOCB_CPU_ZERO
> >  	bool "CPU 0 is a build_forced no-CBs CPU"
> > -	depends on RCU_NOCB_CPU && !NO_HZ_FULL
> > +	depends on RCU_NOCB_CPU && !NO_HZ_FULL_ALL
> >  	help
> >  	  This option forces CPU 0 to be a no-CBs CPU, so that its RCU
> >  	  callbacks are invoked by a per-CPU kthread whose name begins
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index b27b86c7bbfa..17eed0856b03 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -2473,6 +2473,9 @@ static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)
> >  
> >  	if (rcu_nocb_mask == NULL)
> >  		return;
> > +#ifdef CONFIG_NO_HZ_FULL
> > +	cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
> > +#endif /* #ifdef CONFIG_NO_HZ_FULL */
> >  	if (ls == -1) {
> >  		ls = int_sqrt(nr_cpu_ids);
> >  		rcu_nocb_leader_stride = ls;
> > -- 
> > 1.8.1.5
> > 
>