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SubjectRe: [GIT RFC PULL rcu/urgent] Prevent Kconfig from asking pointless questions
On Sat, Apr 18, 2015 at 03:03:41PM +0200, Ingo Molnar wrote:
> * Paul E. McKenney <> wrote:
> > Hello, Ingo,
> >
> > This series contains a single change that fixes Kconfig asking pointless
> > questions ( This is an RFC pull
> > because there has not yet been a -next build for April 16th. If you
> > would prefer to wait until after -next has pulled this, please let me
> > know and I will redo this pull request after that has happened.
> >
> > In the meantime, this change is available in the git repository at:
> >
> > git:// for-mingo
> >
> > for you to fetch changes up to 8d7dc9283f399e1fda4e48a1c453f689326d9396:
> >
> > rcu: Control grace-period delays directly from value (2015-04-14 19:33:59 -0700)
> >
> > ----------------------------------------------------------------
> > Paul E. McKenney (1):
> > rcu: Control grace-period delays directly from value
> >
> > kernel/rcu/tree.c | 16 +++++++++-------
> > lib/Kconfig.debug | 1 +
> > 2 files changed, 10 insertions(+), 7 deletions(-)
> Pulled, thanks a lot Paul!
> Note, while this fixes Linus's immediate complaint that arose from the
> new option, I still think we need to do more fixes in this area.

Good point!

> To demonstrate the current situation I tried the following experiment,
> I did a 'make defconfig' on an x86 box and then took the .config and
> deleted all 'RCU Subsystem' options not marked as debugging.
> Then I did a 'make oldconfig' to see what kinds of questions a user is
> facing when trying to configure RCU:
> *
> * Restart config...
> *
> *
> * RCU Subsystem
> *
> RCU Implementation
> > 1. Tree-based hierarchical RCU (TREE_RCU) (NEW)
> choice[1]: 1

Hmmm... Given that there is no choice, I agree that it is a bit silly
to ask...

> Task_based RCU implementation using voluntary context switch (TASKS_RCU) [N/y/?] (NEW)

Agreed, this one should be driven directly off of CONFIG_RCU_TORTURE_TEST
and the tracing use case.

> Consider userspace as in RCU extended quiescent state (RCU_USER_QS) [N/y/?] (NEW)

This should be driven directly off of CONFIG_NO_HZ_FULL, unless
Frederic knows something I don't.

> Tree-based hierarchical RCU fanout value (RCU_FANOUT) [64] (NEW)

Hmmm... I could drop/obscure this one in favor of a boot parameter.

> Tree-based hierarchical RCU leaf-level fanout value (RCU_FANOUT_LEAF) [16] (NEW)

Ditto -- though large configurations really do set this to 64 in combination
with the skew_tick boot parameter. Maybe we need to drive these off of
some large-system parameter, like CONFIG_MAX_SMP.

> Disable tree-based hierarchical RCU auto-balancing (RCU_FANOUT_EXACT) [N/y/?] (NEW)

I should just make this a boot parameter. Absolutely no reason for it to
be a Kconfig parameter.

> Accelerate last non-dyntick-idle CPU's grace periods (RCU_FAST_NO_HZ) [N/y/?] (NEW)

On this one, I have no idea. Its purpose is energy efficiency, but it
does have some downsides, for example, increasing idle entry/exit latency.
I am a bit nervous about having it be a boot parameter because that
would leave an extra compare-branch in the path. This one will require
some thought.

> Real-time priority to use for RCU worker threads (RCU_KTHREAD_PRIO) [0] (NEW)

Indeed, Linus complained about this one. ;-)

This Kconfig parameter is a stopgap, and needs a real solution. People
with crazy-heavy workloads involving realtime cannot live without it,
but that means that most people don't have to care. I have had solving
this on my list, and this clearly increases its priority.

> Offload RCU callback processing from boot-selected CPUs (RCU_NOCB_CPU) [N/y/?] (NEW)

Hmmm... Maybe a boot parameter, but I thought that there was some reason
that this was problematic. I will have to take another look.

Anyway, this one is important to non-NO_HZ_FULL real-time workloads.
In a -rt kernel, making CONFIG_PREEMPT_RT (or whatever it is these
days) drive this one makes a lot of sense.

> #
> # configuration written to .config
> #
> Only TREE_RCU is available on defconfig, so all the other options
> marked with '(NEW)' were offered as an interactive prompt.
> I don't think that any of the 8 interactive options (!) here are
> particularly useful to even advanced users who configure kernels, and
> I don't think they should be offered under non-expert settings.

Would it make sense to have a CONFIG_RCU_EXPERT setting to hide the
remaining settings? That would reduce the common-case number of
questions to one, which would be a quick and safe improvement.
Especially when combined with the changes I called out above.

> Instead we should pick a preferred RCU configuration based on other
> hints (such as CONFIG_NR_CPUS and CONFIG_NO_HZ settings), and if users
> or distribution makers find some problem with that, we should address
> those specific complaints.
> Making everything under the sun configurable, with which non-RCU
> experts cannot really do anything anyway, isn't very user friendly -
> and results in:
> - user confusion and frustration
> - possibly messed up configurations
> - it also hides inefficiencies that might arise from our defaults:
> someone genuinely finding a problem might just tweak the .config,
> without ever communicating that bad default to us.
> So doing (much!) less is in general the best option for Kconfig driven
> UIs.

I certainly cannot argue with this point!

Thanx, Paul

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