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

[Pranith Kumar <bobby.prani@gmail.com>]

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?

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