[RFC v3 5/8] dmabuf: Add gpu cgroup charge transfer function

[T.J. Mercier]

On Tue, Mar 22, 2022 at 2:52 AM Michal Koutný <mkoutny at suse.com> wrote:
>
> On Mon, Mar 21, 2022 at 04:54:26PM -0700, "T.J. Mercier"
> <tjmercier at google.com> wrote:
> > Since the charge is duplicated in two cgroups for a short period
> > before it is uncharged from the source cgroup I guess the situation
> > you're thinking about is a global (or common ancestor) limit?
>
> The common ancestor was on my mind (after the self-shortcut).
>
> > I can see how that would be a problem for transfers done this way and
> > an alternative would be to swap the order of the charge operations:
> > first uncharge, then try_charge. To be certain the uncharge is
> > reversible if the try_charge fails, I think I'd need either a mutex
> > used at all gpucg_*charge call sites or access to the gpucg_mutex,
>
> Yes, that'd provide safe conditions for such operations, although I'm
> not sure these special types of memory can afford global lock on their
> fast paths.

I have a benchmark I think is suitable, so let me try this change to
the transfer implementation and see how it compares.

>
> > which implies adding transfer support to gpu.c as part of the gpucg_*
> > API itself and calling it here. Am I following correctly here?
>
> My idea was to provide a special API (apart from
> gpucp_{try_charge,uncharge}) to facilitate transfers...
>
> > This series doesn't actually add limit support just accounting, but
> > I'd like to get it right here.
>
> ...which could be implemented (or changed) depending on how the charging
> is realized internally.
>
>
> Michal