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SubjectRe: [GIT RFC PULL rcu/urgent] Prevent Kconfig from asking pointless questions
On Sat, Apr 18, 2015 at 04:32:38PM +0200, Ingo Molnar wrote:
>
> * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
>
> > On Sat, Apr 18, 2015 at 03:03:41PM +0200, Ingo Molnar wrote:
> > >
> > > * Paul E. McKenney <paulmck@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...
>
> To clarify: this doesn't actually ask - it gets skipped by the kconfig
> tool. All the rest is an interactive prompt.

Ah, good point!

> > > 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.
>
> Yeah.

OK, will do.

> > > 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.
>
> Yes.

Then unless Frederic objects... ;-)

> > > Tree-based hierarchical RCU fanout value (RCU_FANOUT) [64] (NEW)
> >
> > Hmmm... I could drop/obscure this one in favor of a boot parameter.
>
> Well, what I think might be even bette to make it scale based on
> CONFIG_NR_CPUS. Distros already actively manage the 'maximum number of
> CPUs we support', so relying on that value makes sense.
>
> So if someone sets CONFIG_NR_CPUS to 1024, it gets scaled accordingly.
> If CONFIG_NR_CPUS is set to 2, it gets scaled to a minimal config.
> Note that this would excercise and test the affected codepaths better
> as well, as we'd get different size setups.
>
> As for the boot option to override it: what would be the usecase for
> that?

Well, in normal circumstances, it should be 64 for 64-bit systems and
32 for 32-bit systems, regardless of number of CPUs. But if you had
an odd-sized multisocket system with extremely high socket-to-socket
memory latencies, you might want to select a different value. For
a silly example, suppose your system had 27 hardware threads per socket.
Then you might want to set both RCU_FANOUT_LEAF and RCU_FANOUT to 27.

Or use a boot parameter to do so, as can be done today for RCU_FANOUT_LEAF.

@@@

> > > 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.
>
> Or rather CONFIG_NR_CPUS. CONFIG_MAX_SMP is really a debugging thing,
> to configure the system to the silliest high settings that doesn't
> outright crash - but it doesn't make much sense otherwise.

Except that setting RCU_FANOUT_LEAF to 64 without also booting with
skew_tick=1 is a really bad idea, as the synchronized scheduling-clock
interrupts will cause ugly levels of lock contention on the rcu_node
->lock. :-(

But perhaps making the default value of sched_skew_tick be 1 if
RCU_FANOUT_LEAF is greater than 16 is the right solution.

> > > 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.
>
> Again I'd size this to NR_CPUS - and for the boot parameter, I'd think
> about actual usecases.

The intended use case is related to the odd-sized systems mentioned
for RCU_FANOUT. By default, we spread CPUs across the leaf-level
rcu_node structures to reduce lock contention, via RCU_FANOUT_EXACT=n.
Systems with high remote memory latencies might want RCU_FANOUT_EXACT=y
to have full control of the geometry.

Maybe I should just eliminate this choice, forcing the current default.

> > > 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.
>
> Keeping this one configurable, with a good default and a good
> explanation makes sense. There's a lot of
>
> > > Real-time priority to use for RCU worker threads (RCU_KTHREAD_PRIO) [0] (NEW)
> >
> > Indeed, Linus complained about this one. ;-)
>
> :-) Yes, it's an essentially unanswerable question.
>
> > 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.
>
> So what value do they use, prio 99? 98? It might be better to offer
> this option as a binary choice, and set a given priority. If -rt
> people complain then they might help us in solving it properly.

I honestly do not remember what priority they were using, it is
not in email, and I don't keep IRC logs that far back. Adding
linux-rt-users@vger.kernel.org on CC.

> > > 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.
>
> Ok.

But in the meantime, it looks like making non-default settings depend on
RCU_EXPERT it the right thing to do.

> > > #
> > > # 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.
>
> Yes, that's absolutely sensible - although I'd also do the
> CONFIG_NR_CPUS based auto-scaling if it's not set, to make sure
> distros don't end up tuning this (inevitably imperfectly) which won't
> flow back upstream:
>
> That's the other main problem with widely tunable, numeric settings,
> beyond their user hostility: if they are wrong and are corrected in a
> distro they don't flow back to upstream, so they are dead end
> mechanisms as far as code quality and good defaults are concerned.

OK, I will put the surviving options under CONFIG_RCU_EXPERT, and I will
check around to see if I can find any cases of distros setting them to
non-default values.

Thanx, Paul



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