[Devel] Re: [PATCH 1/2] rcfs core patch

[Paul Menage]

On 3/7/07, Eric W. Biederman <ebiederm at xmission.com> wrote:
>
> Please next time this kind of patch is posted add a description of
> what is happening and why.  I have yet to see people explain why
> this is a good idea.  Why the current semantics were chosen.

OK. I thought that the descriptions in my last patch 0/7 and
Documentation/containers.txt gave a reasonable amount of "why", but I
can look at adding more details.

>
> I have a question?  What does rcfs look like if we start with
> the code that is in the kernel?  That is start with namespaces
> and nsproxy and just build a filesystem to display/manipulate them?
> With the code built so it will support adding resource controllers
> when they are ready?

There's at least one resource controller that's already in the kernel - cpusets.

> We probably want to rename this struct task_proxy....
> And then we can rename most of the users things like:
> dup_task_proxy, clone_task_proxy, get_task_proxy, free_task_proxy,
> put_task_proxy, exit_task_proxy, init_task_proxy....

That could be a good start.

>
> This extra list of nsproxy's is unneeded and a performance problem the
> way it is used.  In general we want to talk about the individual resource
> controllers not the nsproxy.

There's one important reason why it's needed, and highlights one of
the ways that "resource controllers" are different from the way that
"namespaces" have currently been used.

Currently with a namespace, you can only unshare, either by
sys_unshare() or clone() - you can't "reshare" a namespace with some
other task. But resource controllers tend to have the concept a lot
more of being able to move between resource classes. If you're going
to have an ns_proxy/container_group object that gathers together a
group of pointers to namespaces/subsystem-states, then either:

1) you only allow a task to reshare *all* namespaces/subsystems with
another task, i.e. you can update current->task_proxy to point to
other->task_proxy. But that restricts flexibility of movement. It
would be impossible to have a process that could enter, say, an
existing process' network namespace without also entering its
pid/ipc/uts namespaces and all of its resource limits.

2) you allow a task to selectively reshare namespaces/subsystems with
another task, i.e. you can update current->task_proxy to point to a
proxy that matches your existing task_proxy in some ways and the
task_proxy of your destination in others. In that case a trivial
implementation would be to allocate a new task_proxy and copy some
pointers from the old task_proxy and some from the new. But then
whenever a task moves between different groupings it acquires a new
unique task_proxy. So moving a bunch of tasks between two groupings,
they'd all end up with unique task_proxy objects with identical
contents.

So it would be much more space efficient to be able to locate an
existing task_proxy with an identical set of namespace/subsystem
pointers in that event. The linked list approach that I put in my last
containers patch was a simple way to do that, and Vatsa's reused it
for his patches. My intention is to replace it with a more efficient
lookup (maybe using a hash of the desired pointers?) in a future
patch.

>
> > +     void *ctlr_data[CONFIG_MAX_RC_SUBSYS];
>
> I still don't understand why these pointers are so abstract,
> and why we need an array lookup into them?
>

For the same reason that we have:

- generic notifier chains rather than having a big pile of #ifdef'd
calls to the various notification sites

- linker sections to define initcalls and per-cpu variables, rather
than hard-coding all init calls into init/main.c and having a big
per-cpu structure (both of which would again be full of #ifdefs)

It makes the code much more readable, and makes patches much simpler
and less likely to stomp on one another.

OK, so my current approaches have involved an approach like notifier
chains, i.e. have a generic list/array, and do something to all the
objects on that array.

How about a radically different approach based around the
initcall/percpu way (linker sections)? Something like:

- each namespace or subsystem defines itself in its own code, via a
macro such as:

struct task_subsys {
  const char *name;
  ...
};

#define DECLARE_TASKGROUP_SUBSYSTEM(ss) \
    __attribute__((__section__(".data.tasksubsys"))) struct
task_subsys *ss##_ptr = &ss


It would be used like:

struct taskgroup_subsys uts_ns = {
  .name = "uts",
  .unshare = uts_unshare,
};

DECLARE_TASKGROUP_SUBSYSTEM(uts_ns);

...

struct taskgroup_subsys cpuset_ss {
  .name = "cpuset",
  .create = cpuset_create,
  .attach = cpuset_attach,
};

DECLARE_TASKGROUP_SUBSYSTEM(cpuset_ss);

At boot time, the task_proxy init code would figure out from the size
of the task_subsys section how many pointers had to be in the
task_proxy object (maybe add a few spares for dynamically-loaded
modules?). The offset of the subsystem pointer within the task_subsys
data section would also be the offset of that subsystem's
per-task-group state within the task_proxy object, which should allow
accesses to be pretty efficient (with macros providing user-friendly
access to the appropriate locations in the task_proxy)

The loops in container.c in my patch that iterate over the subsys
array to perform callbacks, and the code in nsproxy.c that performs
the same action for each namespace type, would be replaced with
iterations over the task_subsys data section; possibly some
pre-processing of the various linked-in subsystems could be done to
remove unnecessary iterations. The generic code would handle things
like reference counting.

The existing unshare()/clone() interface would be a way to create a
child "container" (for want of a better term) that shared some
subsystem pointers with its parent and had cloned versions of others
(perhaps only for the namespace-like subsystems?); the filesystem
interface would allow you to create new "containers" that weren't
explicitly associated with processes, and to move processes between
"containers". Also, the filesystem interface would allow you to bind
multiple subsystems together to allow easier manipulation from
userspace, in a similar way to my current containers patch.

So in summary, it takes the concepts that resource controllers and
namespaces share (that of grouping tasks) and unifies them, while not
forcing them to behave exactly the same way. I can envisage some other
per-task pointers that are generally inherited by children being
possibly moved into this in the same way, e.g. task->user and
task->mempolicy, if we could come up with a solution that handles
groupings with sufficiently different lifetimes.

Thoughts?

Paul
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