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

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

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:

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 00dc411..1123ed4 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 (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) */
+
        if (ls == -1) {
                ls = int_sqrt(nr_cpu_ids);
                rcu_nocb_leader_stride = ls;