On Wed, Jul 16, 2014 at 06:38:08PM -0400, Pranith Kumar wrote:
> A kernel built with RCU_NOCB_CPU_ALL build time option will offload callbacks
> from all CPUs. The user cannot override this behavior without recompiling the
> kernel with the RCU_NOCB_CPU_ALL option turned off.
>
> This commit allows the user to override the build-time option by using the
> rcu_nocbs= boot time option without needing to recompile the kernel.
>
> Please note that this is how NO_HZ_FULL_ALL build time option works and this
> commit makes it work similar to that.
>
> Signed-off-by: Pranith Kumar <[email protected]>
I cannot accept this patch. For one thing, tick_nohz_init_all() looks
a bit on the unconditional side when CONFIG_NO_HZ_FULL_ALL=y. For
another thing, we really do not want to be handing the user a tool that
allows CPUs that are nohz_full to not be no-CBs CPUs. For another thing,
if we add this and it turns out to be a bad idea, it will be difficult
to take it back -- someone somewhere will no doubt have scripted the
boot parameter.
Thanx, Paul
> ---
> init/Kconfig | 14 +++++++-------
> kernel/rcu/tree_plugin.h | 8 ++++----
> 2 files changed, 11 insertions(+), 11 deletions(-)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index 41066e4..7d363a4 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -768,13 +768,13 @@ config RCU_NOCB_CPU_ALL
> bool "All CPUs are build_forced no-CBs CPUs"
> depends on RCU_NOCB_CPU
> help
> - This option forces all CPUs to be no-CBs CPUs. The rcu_nocbs=
> - boot parameter will be ignored. All CPUs' RCU callbacks will
> - be executed in the context of per-CPU rcuo kthreads created for
> - this purpose. Assuming that the kthreads whose names start with
> - "rcuo" are bound to "housekeeping" CPUs, this reduces OS jitter
> - on the remaining CPUs, but might decrease memory locality during
> - RCU-callback invocation, thus potentially degrading throughput.
> + If the user doesn't pass the rcu_nocbs= boot option, force all CPUs
> + to be no-CBs CPUs. All CPUs' RCU callbacks will be executed in the
> + context of per-CPU rcuo kthreads created for this purpose. Assuming
> + that the kthreads whose names start with "rcuo" are bound to
> + "housekeeping" CPUs, this reduces OS jitter on the remaining CPUs, but
> + might decrease memory locality during RCU-callback invocation, thus
> + potentially degrading throughput.
>
> Select this if all CPUs need to be no-CBs CPUs for real-time
> or energy-efficiency reasons.
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index a2113f6..b97a939 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -92,15 +92,15 @@ static void __init rcu_bootup_announce_oddness(void)
> if (!have_rcu_nocb_mask) {
> zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL);
> have_rcu_nocb_mask = true;
> +#ifdef CONFIG_RCU_NOCB_CPU_ALL
> + pr_info("\tOffload RCU callbacks from all CPUs\n");
> + cpumask_copy(rcu_nocb_mask, cpu_possible_mask);
> +#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
> }
> #ifdef CONFIG_RCU_NOCB_CPU_ZERO
> pr_info("\tOffload RCU callbacks from CPU 0\n");
> cpumask_set_cpu(0, rcu_nocb_mask);
> #endif /* #ifdef CONFIG_RCU_NOCB_CPU_ZERO */
> -#ifdef CONFIG_RCU_NOCB_CPU_ALL
> - pr_info("\tOffload RCU callbacks from all CPUs\n");
> - cpumask_copy(rcu_nocb_mask, cpu_possible_mask);
> -#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
> #endif /* #ifndef CONFIG_RCU_NOCB_CPU_NONE */
> if (have_rcu_nocb_mask) {
> if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
> --
> 1.9.1
>
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