Re: [PATCH v8 -tip 19/26] sched: Add a second-level tag for nested CGroup usecase

[Joel Fernandes]

On Wed, Oct 28, 2020 at 02:23:02PM +0800, Li, Aubrey wrote:
> On 2020/10/20 9:43, Joel Fernandes (Google) wrote:
> > Google has a usecase where the first level tag to tag a CGroup is not
> > sufficient. So, a patch is carried for years where a second tag is added which
> > is writeable by unprivileged users.
> > 
> > Google uses DAC controls to make the 'tag' possible to set only by root while
> > the second-level 'color' can be changed by anyone. The actual names that
> > Google uses is different, but the concept is the same.
> > 
> > The hierarchy looks like:
> > 
> > Root group
> >    / \
> >   A   B    (These are created by the root daemon - borglet).
> >  / \   \
> > C   D   E  (These are created by AppEngine within the container).
> > 
> > The reason why Google has two parts is that AppEngine wants to allow a subset of
> > subcgroups within a parent tagged cgroup sharing execution. Think of these
> > subcgroups belong to the same customer or project. Because these subcgroups are
> > created by AppEngine, they are not tracked by borglet (the root daemon),
> > therefore borglet won't have a chance to set a color for them. That's where
> > 'color' file comes from. Color could be set by AppEngine, and once set, the
> > normal tasks within the subcgroup would not be able to overwrite it. This is
> > enforced by promoting the permission of the color file in cgroupfs.
> > 
> > The 'color' is a 8-bit value allowing for upto 256 unique colors. IMHO, having
> > more than these many CGroups sounds like a scalability issue so this suffices.
> > We steal the lower 8-bits of the cookie to set the color.
> > 
> 
> So when color = 0, tasks in group A C D can run together on the HTs in same core,
> And if I set the color of taskC in group C = 1, then taskC has a different cookie
> from taskA and taskD, so in terms of taskA, what's the difference between taskC
> and [taskB or taskE]? The color breaks the relationship that C belongs to A.

C does belong to A in the sense, A cannot share with B, this implies C can
never share with B. Setting C's color does not change that fact. So coloring
is irrelevant in your question.

Sure, A cannot share with C either after coloring, but that's irrelevant and
not the point of doing the coloring.

thanks,

 - Joel