On Sat, Apr 18, 2015 at 03:03:41PM +0200, Ingo Molnar wrote:
>
> * Paul E. McKenney <paul...@linux.vnet.ibm.com> wrote:
>
> > Hello, Ingo,
> >
> > This series contains a single change that fixes Kconfig asking pointless
> > questions (https://lkml.org/lkml/2015/4/14/616). 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://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.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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