linux-kernel - Re: [GIT PULL rcu/next] RCU commits for 3.17

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Date:	Wed, 16 Jul 2014 11:35:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	fabf@...net.be, davidshan@...cent.com, joe@...ches.com,
	keescook@...omium.org, Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [GIT PULL rcu/next] RCU commits for 3.17

On Wed, Jul 16, 2014 at 02:24:55PM -0400, Pranith Kumar wrote:
> On 07/16/2014 02:15 PM, Pranith Kumar wrote:
> >
> > On 07/16/2014 01:24 PM, Paul E. McKenney wrote:
> >>>> And of course if you don't actually specify a nohz_full= mask, then
> >>>> tick_nohz_full_mask can be NULL at RCU initialization time, and if it
> >>>> is also true that CONFIG_NO_HZ_FULL_ALL=n, this condition can persist
> >>>> forever.
> >>>>
> >>> Hi Paul,
> >>>
> >>> The other location where tick_nohz_full_mask is being allocated is in
> >>> tick_nohz_init_all(), called from tick_nohz_init(). rcu_init() is called before
> >>> tick_nohz_init() in init/main.c. CONFIG_NO_HZ_FULL_ALL for allocation of the
> >>> mask does not take effect when rcu_init() runs.
> >>>
> >>> So if nohz_full command line arg is not specified, tick_nohz_full_mask will
> >>> always be NULL when rcu_init() runs. So just checking for tick_nohz_full_mask
> >>> is NULL should be enough I guess.
> >> Yep, that is indeed one of the conditions called out in the commit log
> >> below.
> >>
> >>                             Thanx, Paul
> >>
> >
> > Sorry, I was not clear.
> >
> > nohz_full= boot time parameter overrides the CONFIG_NO_HZ_FULL_ALL build time parameter.
> > The check for !CONFIG_NO_HZ_FULL_ALL will disable masking out the cpus on which NOHZ is
> > not enabled using the nohz_full boot time parameter, and hence you will enable callback
> > offloading on all CPUs as was previously being done.
> >
> > So, if I have CONFIG_NO_HZ_FULL_ALL enabled at build time but pass nohz_cpu=1 at boot
> > time, all the CPUs will have callback offloading enabled.
> >
> > I think the following looks like what is needed in our case:
> >
> 
> The previous patch will fail if CONFIG_NO_HZ_FULL is not defined, this should work.
> 
> Thoughts?

If CONFIG_NO_HZ_FULL_ALL=y, then CONFIG_RCU_NOCB_CPU_ALL=y, in which case
the cpumask_or() is not required because all the bits will already be
set in rcu_nocb_mask.  So no need to change the #if condition.  And
if we were to change it, we would hopefully change it to #ifdef in
this particular case.  ;-)

							Thanx, Paul

> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 00dc411..83f3f58 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2479,10 +2479,10 @@ static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)
> 
>         if (rcu_nocb_mask == NULL)
>                 return;
> -#if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL)
> +#if defined(CONFIG_NO_HZ_FULL)
>         if (tick_nohz_full_running)
>                 cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
> -#endif /* #if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL) */
> +#endif /* #if defined(CONFIG_NO_HZ_FULL) */
>         if (ls == -1) {
>                 ls = int_sqrt(nr_cpu_ids);
>                 rcu_nocb_leader_stride = ls;
> 
> --
> Pranith
> 
> 
> 
> 
> 

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