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rcu: Don't offload callbacks unless specifically requested

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Enabling NO_HZ_FULL currently has the side effect of enabling callback
offloading on all CPUs.  This results in lots of additional rcuo kthreads,
and can also increase context switching and wakeups, even in cases where
callback offloading is neither needed nor particularly desirable.  This
commit therefore enables callback offloading on a given CPU only if
specifically requested at build time or boot time, or if that CPU has
been specifically designated (again, either at build time or boot time)
as a nohz_full CPU.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2014-07-02 10:13:24 -07:00
parent fbce7497ee
commit b58cc46c5f
2 changed files with 5 additions and 2 deletions
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4
init/Kconfig
View File

@ -737,7 +737,7 @@ choice
config RCU_NOCB_CPU_NONE
bool "No build_forced no-CBs CPUs"
depends on RCU_NOCB_CPU && !NO_HZ_FULL
depends on RCU_NOCB_CPU && !NO_HZ_FULL_ALL
help
This option does not force any of the CPUs to be no-CBs CPUs.
Only CPUs designated by the rcu_nocbs= boot parameter will be
@ -751,7 +751,7 @@ config RCU_NOCB_CPU_NONE
config RCU_NOCB_CPU_ZERO
bool "CPU 0 is a build_forced no-CBs CPU"
depends on RCU_NOCB_CPU && !NO_HZ_FULL
depends on RCU_NOCB_CPU && !NO_HZ_FULL_ALL
help
This option forces CPU 0 to be a no-CBs CPU, so that its RCU
callbacks are invoked by a per-CPU kthread whose name begins

3
kernel/rcu/tree_plugin.h
View File

@ -2473,6 +2473,9 @@ static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)
if (rcu_nocb_mask == NULL)
return;
#ifdef CONFIG_NO_HZ_FULL
cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
#endif /* #ifdef CONFIG_NO_HZ_FULL */
if (ls == -1) {
ls = int_sqrt(nr_cpu_ids);
rcu_nocb_leader_stride = ls;