[LAU] More recent kernel config options

In addition to the question I posted earlier about PREEMPT_RCU, I've
found still other kernel config options that are not covered in most of
the extant howtos for setting up low latency kernels, because they are
recently added in the unpatched kernel. (Or at least, they're more
recent than most of the howtos...)

I've been researching these options, mostly by googling (and
googling...and googling) the Linux Kernel Mailing List archives, and
also by looking at the config used by 64Studio, since it seems the
prevalent opinion is that they are stable as a rock. With the latter
approach of checking against 64Studio's config, I've had little luck
though, because their distro currently comes in a 2.1 "stable" version
that is based on Debian "etch", with a 2.6.21 kernel that predates the
existence of some of the config options I'm asking about; and a 3.0
"beta" version that uses a newer kernel, but should also probably be
taken with a grain of salt, because 64Studio is still working the bugs
out of it, and because it seems to have options turned on that the LKML
archives have warned strongly against. So, most of the information I've
gleaned has been from the LKML.

I might as well collate these questions here, for what good it may do.
The kernel I'm configuring is 2.6.29.6 with matching RT patches (-rt23),
though all of the options listed here are actually present even in the
unpatched 2.6.29.6 kernel.

PREEMPT_RCU :

This was the one my original question was about in the earlier post.
In the newer kernels (even mainline ones, without any RT patches), there
is a choice of "RCU Subsystem", with one option being "classic", and
other being "preemptable". The choice would seem obvious for low
latency, except that the help text warns that a preemptible RCU is
likely to expose serious kernel bugs that may render the system
completely unstable. (This is *not* the same setting as the various
CONFIG_PREEMPT options that have been present in the mainline kernel for
awhile now. This is something new, and it appears in menuconfig as a
separate setting.) I think I've mostly gathered from reading through
the whole process of debugging that the kernel gurus went through that
as of last year or so, this is now considered mostly stable, after
having been subjected to a utility called 'rcutorture', and having fixed
many lockups. I'd still be interested in anything anyone else can
contribute about it though, especially if they're using it and they
also think it is stable.

GROUP_SCHED :

This one is interesting, and I don't know what to make of it, other than
that the LKML seems to have decided in the last two months or so that it
slows your system down and makes latencies worse.
         The thing that's confusing about it though is that it's
described as a mechanism for grouping high priority tasks by group.
It's implied (though not spelled out specifically) that they even mean
by this Posix groups, because in one document I read, it says that
enabling this will cause you to be unable to get realtime as a non-root
user unless you are setup in a group specified in limits.conf. Hmm!
That sounds an awful lot like what we've just been calling pam_limits
for years now. Are we doing this with a kernel config option now? (One
which apparently doesn't work?)
         The 3.0 (beta) version of 64Studio turns this on. Then again,
looking at their kernel config, they seem to turn everything on, and
that might be why it's considered beta. (The 2.1 stable version of
64Studio seems to have a kernel old enough that it never had that
option as a choice.) Anyone have any feedback about this? It's only in
the past two months that the kernel gurus have decided that it's bad,
but they're actually considering marking it broken from what I've read.

CGROUPS :

This sounds cool, but I'm reasonably sure it's not actually necessary.
Documentation I've read suggests that this allows for letting
applications (or users) define CPU and memory pool affinity for tasks,
so that one could arbitrarily tie down particular threads or tasks to a
given processor core, or region of memory (or something like that).
However, the thing that makes me somewhat sure I don't positively need
this is that the same documentation also says that if you want to use
it, you need to create a new subdirectory under /dev, mount a new
pseudofilesystem under it, and then this module will populate that space
with dynamic configuration data about these affinity groups for running
tasks. I have neither seen any distro (even ones made for musicians)
that has set any such thing up out of the box, nor have I ever seen a
realtime howto that tells people to do it themselves. It sounds like
there's a lot of infrastructure necessary for this that's not common in
distros yet. (Though I see that regular Debian "lenny" turns this on in
the kernel without actually providing the special /dev support for it,
which I presume they're thinking you'll setup yourself if you're
interested.) Still, comments welcome...

-- 
+ Brent A. Busby	 + "We've all heard that a million monkeys
+ UNIX Systems Admin	 +  banging on a million typewriters will
+ University of Chicago	 +  eventually reproduce the entire works of
+ Physical Sciences Div. +  Shakespeare.  Now, thanks to the Internet,
+ James Franck Institute +  we know this is not true." -Robert Wilensky
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