[RFC PATCH v3 04/11] drm, cgroup: Add total GEM buffer allocation limit

[Kenny Ho]

On Thu, Jun 27, 2019 at 5:24 PM Daniel Vetter <daniel at ffwll.ch> wrote:
> On Thu, Jun 27, 2019 at 02:42:43PM -0400, Kenny Ho wrote:
> > Um... I am going to get a bit philosophical here and suggest that the
> > idea of sharing (especially uncontrolled sharing) is inherently at odd
> > with containment.  It's like, if everybody is special, no one is
> > special.  Perhaps an alternative is to make this configurable so that
> > people can allow sharing knowing the caveat?  And just to be clear,
> > the current solution allows for sharing, even between cgroup.
>
> The thing is, why shouldn't we just allow it (with some documented
> caveat)?
>
> I mean if all people do is share it as your current patches allow, then
> there's nothing funny going on (at least if we go with just leaking the
> allocations). If we allow additional sharing, then that's a plus.
Um... perhaps I was being overly conservative :).  So let me
illustrate with an example to add more clarity and get more comments
on it.

Let say we have the following cgroup hierarchy (The letters are
cgroups with R being the root cgroup.  The numbers in brackets are
processes.  The processes are placed with the 'No Internal Process
Constraint' in mind.)
R (4, 5) ------ A (6)
  \
    B ---- C (7,8)
     \
       D (9)

Here is a list of operation and the associated effect on the size
track by the cgroups (for simplicity, each buffer is 1 unit in size.)
With current implementation (charge on buffer creation with
restriction on sharing.)
R   A   B   C   D   |Ops
================
1   0   0   0   0   |4 allocated a buffer
1   0   0   0   0   |4 shared a buffer with 5
1   0   0   0   0   |4 shared a buffer with 9
2   0   1   0   1   |9 allocated a buffer
3   0   2   1   1   |7 allocated a buffer
3   0   2   1   1   |7 shared a buffer with 8
3   0   2   1   1   |7 sharing with 9 (not allowed)
3   0   2   1   1   |7 sharing with 4 (not allowed)
3   0   2   1   1   |7 release a buffer
2   0   1   0   1   |8 release a buffer from 7

The suggestion as I understand it (charge per buffer reference with
unrestricted sharing.)
R   A   B   C   D   |Ops
================
1   0   0   0   0   |4 allocated a buffer
2   0   0   0   0   |4 shared a buffer with 5
3   0   0   0   1   |4 shared a buffer with 9
4   0   1   0   2   |9 allocated a buffer
5   0   2   1   1   |7 allocated a buffer
6   0   3   2   1   |7 shared a buffer with 8
7   0   4   2   2   |7 sharing with 9
8   0   4   2   2   |7 sharing with 4
7   0   3   1   2   |7 release a buffer
6   0   2   0   2   |8 release a buffer from 7

Is this a correct understanding of the suggestion?

Regards,
Kenny