On Tue, 2010-11-30 at 14:36 -0500, Vivek Goyal wrote:

quoted text
> Few things.
> 
> - This /proc/<pid>/autogroup is good for doing this single thing but when
>   I start thinking of possible extensions of it down the line, it creates
>   issues. 
> 
> - Once we have some kind of uppper limit support in cpu controller, these
>   autogroups are beyond control. If you want to impose some kind of 
>   limits on them then you shall have to extend parallel interface
>   /proc/<pid>/autogroup to also speicify upper limit (like nice levels).
> 
> - Similiarly if this autgroup notion is extended to other cgroup
>   controllers, then you shall have to again extend /proc/<pid>/autogroup
>   to be able to specify these additional parameters.

Yes, if it evolves, it's interface will need to evolve as well.  It
could have been a directory containing buttons, knobs and statistics,
but KISS won.

quoted text
> - If there is a monitoring tool which is monitoring the system for
>   resource usage by the groups, then I think these autogroups are beyond
>   reach and any stats exported by cgroup interface will not be available.
>   (though right now I can't see any stats being exported by cgroup files
>    in cpu controller but other controllers like block and memory do.).

If you're manually assigning bandwidth et al from userland, there's not
much point to in-kernel automation is there?

If I had married the two, the first thing that would have happened is
gripes about things appearing and disappearing in cgroups directories,
resulting in mayhem and confusion for scripts and tools.

quoted text
> - I am doing some testing with the patch and w.r.t. cgroup interface some
>   things don't seem right.
> 
>   I have applied your patch and enabled CONFIG_AUTO_GROUP. Now I boot
>   into the kernel and open a new ssh connection to the machine. 
> 
>   # echo $$
>     3555
>   # cat /proc/3555/autogroup
>     /autogroup-63 nice 0
> 
>   IIUC, task 3555 has been moved into an autogroup. Now I mount the cpu
>   controller and this task is visible in root cgroup.
> 
>   # mount -t cgroup -o cpu none /cgroup/cpu
>   # cat /cgroup/cpu/tasks | grep 3555
>     3555
> 
>   First of all this gives user a wrong impression that task 3555 is in
>   root cgroup.

It is in the root cgroup.  It is not in the root autogroup is not
auto-cgroups group.

quoted text
>   Now I create a child group test1 and move the task there and also change
>   the weight/shares of the cgroup to 10240.
> 
>   # mkdir test1
>   # echo 3555 > test1/tasks
>   # echo 10240 > test1/cpu.shares
>   # cat /proc/3555/cgroup
>     3:cpu:/test1
>   # cat /proc/3555/autogroup
>     /autogroup-63 nice 0
> 
> So again, user will think that task is in cgroup test1 and is being
> controlled by the respective weight but that's not the case.

It is the case here.

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  P COMMAND
 7573 root      20   0  7996  340  256 R   50  0.0   3:35.86 3 pert
 7572 root      20   0  7996  340  256 R   50  0.0   9:21.68 3 pert
...
marge:/cgroups/test # echo 7572 > tasks
marge:/cgroups/test # echo 4096 > cpu.shares

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  P COMMAND
 7572 root      20   0  7996  340  256 R   80  0.0  10:06.92 3 pert
 7573 root      20   0  7996  340  256 R   20  0.0   4:05.80 3 pert

When you move a task into a cgroup, it still has an autogroup
association, as all tasks (processes actually) do, but it's not used.

quoted text
> Even if we prevent autogroup task from being visible in cpu controller
> root group, then comes the question what happens if cpu and some other
> controller is comounted. Say cpuset. Now in that case will task be 
> visible in root group task file and can one operate on that. Now showing
> up there does not make much sense as task should still be controllable
> by other controllers and its policies.

The user has to specifically ask for it in his config, can turn it on or
off on the fly or at boot..

quoted text
> So yes, creating a /proc/<pid>/autogroup is dirt cheap and makes the life
> easier in terms of implementation of this patch and it should work well.
> But it is also a new user interface which does not sound too extensible and
> does not seem to cooperate well with cgroup interface

..it has a different mission, with different users being targeted, so
why does it need to hold hands?

quoted text
> It also introduces this new notion of niceness for task groups which is sort
> of equivalent to cpu.shares in cpu controller. First of all why should we
> not stick to shares notion even for autogroup. Even if we introduce the notion
> of niceness for groups, IMHO, it should be through cgroup interface instead of
> group niceness for autogroup and shares/weights for cgroup despite the
> fact that in the background they do similar things.

IMHO, cgroups should have been 'nice' from the start, but the folks who
wrote it did what they thought best.  I like nice a lot better than
shares, so I used nice.

quoted text
> I think above concerns can possibly be reason enough to think about about
> the wedding.

Perhaps in future, they'll get married, and perhaps they should, but in
the here and now, I think they have similar but not identical missions.
If you turn on one, turn off the other.  Maybe that should be automated.

Systemd thingy may make autogroup short lived anyway.  I had a query
from an embedded guy (hm, which I spaced) suggesting autogroup may be
quite nice for handheld stuff though, so who knows.

	-Mike

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