Messages in this thread | | | Date | Wed, 16 Jul 2014 11:32:55 -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 02:15:50PM -0400, 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: > > 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;
Agreed, and I queued something very similar to this. Testing is in progress. If it works, I will send it out. If you want to see it earlier, it is at branch rcu/urgent on the -rcu tree.
Thanx, Paul
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