On Sun, 2016-03-13 at 11:00 -0400, Tejun Heo wrote:

Hello, Mike.

On Sat, Mar 12, 2016 at 07:26:59AM +0100, Mike Galbraith wrote:

quoted

Hrm.  You're showing that per-thread groups can coexist just fine,
which is good given need and usage exists today out in the wild.  Why
do such groups have to be invisible with a unique interface though?

I tried to explain these in the forementioned RFD document.  I'll give
a brief example here.

Let's say there is an application which wants to manage resource
distributions across its multiple threadpools in a hierarchical way.
With cgroupfs interface as the only system-wide interface, it has to
coordinate who or whatever is managing that interface.  Maybe it can
get a subtree delegated to it, maybe it has to ask the system thing to
create and place threads there, maybe it can just expose the pids and
let the system management do its thing (what if the threads in the
pools are dynamic tho?).  There is no reliable universal way of doing
this.  Each such application has to be ready to specifically
coordinate with the specific system management in use.

The last thing I ever want to see on my boxen is random applications
either doing their own thing with my cgroups management interface, or
conspiring with "the system thing" behind my back to do things that I
did not specifically ask them to do.

"The system thing" started doing its own thing behind my back, and
oddly enough, its tentacles started falling off.  By golly, its eyes
seem to have fallen out as well.

That's what happens when control freak meets control freak, one of them
ends up in pieces.  There can be only one, and that one is me, the
administrator.  Applications don't coordinate spit, if I put on my
administrator hat and stuff 'em in a box, they better stay stuffed.

This is kernel failing to provide proper abstraction and isolation
between different layers.  The "raw" feature is there but it's unsafe
to use and thus can't be used widely.

Good, management of my boxen is my turf.  The raw feature works fine
today, and I'd like to see it to keep on working tomorrow.  If tools
written for administration diddle, that's fine IFF I'm the guy on the
other end of the tool.  All other manipulators of my management
interfaces can go.. fish.

quoted

Given the core has to deal with them whether they're visible or not,
and given they exist to fulfill a need, seems they should be first
class citizens, not some Quasimodo like creature sneaking into the
cathedral via a back door and slinking about in the shadows.

In terms of programmability and accessibility for individual
applications, group resource management being available through
straight-forward and incremental extension of exsiting mechanisms is
*way* more first class citizen.  It is two seamless extensions to
clone(2) and setpriority(2) making hierarchical resource management
generally available to applications.

To me, that sounds like chaos.

There can be use cases where building cpu resource hierarchy which is
completely alien to how the rest of the system is organized is useful.
For those cases, the only thing which can be done is building a
separate hierarchy for the cpu controller and that capability isn't
going anywhere.

As long as administrators can use the system interface to aggregate
what they see fit, I'm happy.  The scheduler schedules threads, ergo
the cpu controller must aggregate threads.  There is no process.

	-Mike