[Devel] Re: [PATCH 03/33] task containersv11 add tasks file interface

[Paul Menage]

On 10/3/07, Paul Jackson <pj at sgi.com> wrote:
>
>  - What are these apparent 'exec notifications' that are provided to
>    user space that the following mentions - I cannot find any other
>    mention of them:
>
>         With the ability to classify tasks differently for different
>         resources (by putting those resource subsystems in different
>         hierarchies) then the admin can easily set up a script which
>         receives exec notifications and depending on who is launching
>         the browser he can

It's the process connector netlink notifier. It can report
fork/exit/exec/setuid events to userspace. See
drivers/connector/cn_proc.c

>
>
>  -  It states in cgroups.txt:
>
>     *** notify_on_release is disabled in the current patch set. It will be
>     *** reactivated in a future patch in a less-intrusive manner
>
>     This doesn't seem to be true, and had better not be true.
>     From what I can tell, notify_on_release still works for cpusets,
>     and it is important that it continue to work when cgroups are
>     folded into the main line kernel.

Correct, it's reactivated in a later patch in the series, but this
intermediate comment snuck through.

>
>         Each cgroup object created by the system has an array of pointers,
>         indexed by subsystem id; this pointer is entirely managed by the
>         subsystem; the generic cgroup code will never touch this pointer.
>
>   Is plural "pointers", or singular "pointer", the correct wording?

Probably plural.

>
>  - Several lines near the end of cgroups.txt start with "LL".
>    I guess they list what locks are held while taking the call,
>    but the notation seems cryptic and unfamiliar to me, and its
>    meaning here undocumented.

"Locking Level", describing which locks *are* held, and which are
*not* held during a call. I thought it was a more generally
widely-used commenting convention, but I don't see any other uses of
it in the kernel. I can replace them with "holds cgroup_mutex" or
"doesn't hold cgroup_mutex" for clarity.

>
>  - There are many instances of the local variable 'cont', referring
>    to a struct cgroup pointer.  I presume the spelling 'cont' is a
>    holdover from the time when we called these containers.

Yes, and since cgroup is short for "control group", "cont" still
seemed like a reasonable abbreviation. (And made the automatic
renaming much simpler).
>
>  - The code in attach_task which skips the attachment of a task to
>    the group it is already in has to be removed.  Cpusets depends
>    on reattaching a task to its current cpuset, in order to trigger
>    updating the cpus_allowed mask in the task struct.  This is a
>    hack, granted, but an important one.  It avoids checking for a
>    changed cpuset 'cpus' setting in critical scheduler code paths.

I don't quite understand how this is meant to work - under what
circumstances would it occur? Are there cases when userspace is
required to try to reattach a task to its current cpuset in order to
get a cpu mask change to stick?

Other comments noted, thanks.

Paul
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