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Commit b58cc46c authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Don't offload callbacks unless specifically requested 



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: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent fbce7497

init/Kconfig

+2 −2
Original line number Diff line number Diff line
@@ -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

kernel/rcu/tree_plugin.h

+3 −0
Original line number Diff line number Diff line
@@ -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;