On Fri, Apr 15, 2022 at 10:35:15AM -0700, Paul E. McKenney wrote:
> On Fri, Apr 15, 2022 at 04:02:24PM +0000, Joel Fernandes (Google) wrote:
> > From: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
>
> Much better, thank you!
>
> > On systems with CONFIG_RCU_NOCB_CPU=y, there is no default mask provided
> > which ends up not offloading any CPU. This patch removes a dependency
> > from the bootloader having to know about RCU, about how many CPUs the
> > system has, and about how to provide the mask.
>
> The "about how many CPUs the system has" does not apply to current
> mainline, in which "rcu_nocbs=0-N" says to offload all CPUs. It can be
> added back in for a backport to v5.10. ;-)
True, will fix.
> My thought is to queue this after some independent testing.
>
> > ---
> > v2 was forcing the option to override no_cbs=
> > v3 is back to v1 but with a config option defaulting to 'n'.
> >
> > Documentation/admin-guide/kernel-parameters.txt | 3 +++
> > kernel/rcu/Kconfig | 13 +++++++++++++
> > kernel/rcu/tree_nocb.h | 16 ++++++++++++++--
> > 3 files changed, 30 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index f5a27f067db9..7648a7dd335e 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -4398,6 +4398,9 @@
> > no-callback mode from boot but the mode may be
> > toggled at runtime via cpusets.
> >
> > + Note that this argument takes precedence over
> > + the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option.
>
> Very good, thank you!
>
> Do we want to say the same about nohz_full? Or am I misreading the
> code below?
You're right, will fix.
> > +
> > rcu_nocb_poll [KNL]
> > Rather than requiring that offloaded CPUs
> > (specified by rcu_nocbs= above) explicitly
> > diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> > index bf8e341e75b4..2f8bd694ed85 100644
> > --- a/kernel/rcu/Kconfig
> > +++ b/kernel/rcu/Kconfig
> > @@ -223,6 +223,19 @@ config RCU_NOCB_CPU
> > Say Y here if you need reduced OS jitter, despite added overhead.
> > Say N here if you are unsure.
> >
> > +config RCU_NOCB_CPU_DEFAULT_ALL
> > + bool "Offload RCU callback processing from all CPUs by default"
> > + depends on RCU_NOCB_CPU
>
> The needed dependency on RCU_EXPERT is provided transitively via
> RCU_NOCB_CPU, so this should be OK. (To check, build a .config file,
> queue this patch, and do "make oldconfig". If any questions are asked,
> a change is needed.)
Ok. I did that and no questions were asked.
> > + default n
> > + help
> > + Use this option to offload callback processing from all CPUs
> > + by default, in the absence of the rcu_nocbs boot parameter.
>
> And also in the absence of the nohz_full boot parameter, correct?
Yes, fixed.
> > + This also avoids the need to use any boot parameters to achieve
> > + the effect of offloading all CPUs on boot.
> > +
> > + Say Y here if you want offload all CPUs by default on boot.
> > + Say N here if you are unsure.
> > +
> > config TASKS_TRACE_RCU_READ_MB
> > bool "Tasks Trace RCU readers use memory barriers in user and idle"
> > depends on RCU_EXPERT
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index eeafb546a7a0..673fa0d1f801 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -1165,12 +1165,21 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
> > void __init rcu_init_nohz(void)
> > {
> > int cpu;
> > - bool need_rcu_nocb_mask = false;
> > + bool need_rcu_nocb_mask = false, offload_all = false;
>
> Please use the extra line to perserve alphabetical order. (I do know
> about inverse tree, though I have no idea why it is considered to be a
> good thing.)
Done.
thanks,
- Joel
