Messages in this thread Patch in this message | | | Date | Wed, 16 Jul 2014 14:15:50 -0400 | From | Pranith Kumar <> | Subject | Re: [GIT PULL rcu/next] RCU commits for 3.17 |
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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;
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