Messages in this thread | | | Date | Wed, 16 Jul 2014 11:35:39 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [GIT PULL rcu/next] RCU commits for 3.17 |
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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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