Messages in this thread | | | Date | Wed, 16 Jul 2014 17:26:51 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [RFC PATCH 1/1] rcu: Allow user to ovveride RCU_NOCB_CPU_ALL at boot time |
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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 <bobby.prani@gmail.com>
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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