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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