rcu: Don't offload callbacks unless specifically requested (b58cc46c) · Commits · 戴 / test

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>

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;

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