[Devel] Re: [PATCH, v3 2/2] cgroups: introduce timer slack subsystem

[Jacob Pan]

On Mon, 7 Feb 2011 13:06:03 +0200
"Kirill A. Shutemov" <kirill at shutemov.name> wrote:

> On Fri, Feb 04, 2011 at 09:27:55AM -0800, Jacob Pan wrote:
> > On Fri, 4 Feb 2011 15:34:39 +0200
> > "Kirill A. Shutemov" <kirill at shutemov.name> wrote:
> > > What's mean "original timer slack" if you are free to move a task
> > > between a lot of cgroups and process itself free to change it
> > > anytime?
> > > 
> > 
> > I need to manage tasks by a management software instead of letting
> > the task change timer_slack by itself. The goal is to make
> > management transparent and no modifications to the existing apps.
> > Therefore, it is desirable to automatically enforce timer_slack
> > when the apps are in the cgroup while automatically restore it when
> > it is no longer under cgroup management.
> 
> Tasks are always under cgroup management. Root cgroup is still cgroup.
> 
> > So the "original timer slack" can be the default 50us or whatever
> > value chosen by the task itself. But the app itself should not care
> > or even be aware of which cgroup it is in.
> > 
> > So here are two optoins i can think of
> > 1. add a new variable called cg_timer_slack_ns to struct
> > task_struct{} cg_timer_slack_ns will be set by cgroup timer_slack
> > subsystem, then we can retain the original per task value in
> > timer_slack_ns. timer code will pick max(cg_timer_slack_ns,
> > timer_slack_ns) if cg_timer_slack_ns is set.
> > 
> > 2. leave task_struct unchanged, add a current_timer_slack to the
> > cgroup. timer_slack cgroup does not modify per task timer_slack_ns.
> > similar to option #1, let timer code pick the timer_slack to use
> > based on whether the task is in timer_slack cgroup.
> > 
> > Any thoughts?
> 
> I think it's over-engineering.
> 
We do have a real use case that cannot be solved by the current logic,
I only want to find a solution.
> What about configuration like this:
> 
>  root_cgroup
>  |--timer_slack.min_slack_ns = 0
>  |--timer_slack.max_slack_ns = ULONG_MAX
>  |--50us
>  |  |--timer_slack.min_slack_ns = 50000
>  |  |--timer_slack.max_slack_ns = 50000
>  |--500us
>     |--timer_slack.min_slack_ns = 500000
>     |--timer_slack_max_slack_ns = ULONG_MAX
> 
> If you want a task allow to drive its timer_slack, just leave it in
> root_cgroup.
> 
> It you want to drive timer_slack of a task, move it between 50us and
> 500um based on your policy.
>  
I surely agree with the dummy root configuration. But when the
management software moves task among 50us or 500us cgroups, or back to
dummy root, the timer slack cannot be automatically changed.

consider the following scenarios.
1. task_A has timer_slack = ts1 = 3us when it is running in the
foreground by window manager
2. then it is moved to 50us cgroup because it is no longer in the
foreground, so now ts1 = 50us. 
3. After a while, the system is running in the low power state, so
task_A is moved to 500us cgroup, ts1 = 500us. Then the user switch the
device into normal running state and put task_A in foreground again.
4. Management software then moves task_A from 500us cgroup to dummy
root, but it will not be able to restore the 3us timer slack as needed
by task_A. Task can surely drive its timer_slack but it breaks the
cgroup-transparency desired by the management scheme.

The key is that tasks being managed are not aware of cgroup
involvement. The management software is the one that moves tasks
around, but we don't want the management software keeps track of very
timer slacks value of every tasks. 

Thanks,
Jacob
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