Re: [RFC 4/6] dma-buf: Add DMA-BUF exporter op to charge a DMA-BUF to a cgroup.

[Hridya Valsaraju <hridya@google.com>]

On Wed, Jan 19, 2022 at 7:58 AM Christian König
<christian.koenig@amd.com> wrote:
>
> Am 19.01.22 um 16:54 schrieb Daniel Vetter:
> > On Tue, Jan 18, 2022 at 10:54:16AM -0800, Hridya Valsaraju wrote:
> >> On Sun, Jan 16, 2022 at 11:46 PM Christian König
> >> <christian.koenig@amd.com> wrote:
> >>> Am 15.01.22 um 02:06 schrieb Hridya Valsaraju:
> >>>> The optional exporter op provides a way for processes to transfer
> >>>> charge of a buffer to a different process. This is essential for the
> >>>> cases where a central allocator process does allocations for various
> >>>> subsystems, hands over the fd to the client who
> >>>> requested the memory and drops all references to the allocated memory.
> >>>>
> >>>> Signed-off-by: Hridya Valsaraju <hridya@google.com>
> >>>> ---
> >>>>    include/linux/dma-buf.h | 18 ++++++++++++++++++
> >>>>    1 file changed, 18 insertions(+)
> >>>>
> >>>> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> >>>> index 7ab50076e7a6..d5e52f81cc6f 100644
> >>>> --- a/include/linux/dma-buf.h
> >>>> +++ b/include/linux/dma-buf.h
> >>>> @@ -13,6 +13,7 @@
> >>>>    #ifndef __DMA_BUF_H__
> >>>>    #define __DMA_BUF_H__
> >>>>
> >>>> +#include <linux/cgroup_gpu.h>
> >>>>    #include <linux/dma-buf-map.h>
> >>>>    #include <linux/file.h>
> >>>>    #include <linux/err.h>
> >>>> @@ -285,6 +286,23 @@ struct dma_buf_ops {
> >>>>
> >>>>        int (*vmap)(struct dma_buf *dmabuf, struct dma_buf_map *map);
> >>>>        void (*vunmap)(struct dma_buf *dmabuf, struct dma_buf_map *map);
> >>>> +
> >>>> +     /**
> >>>> +      * @charge_to_cgroup:
> >>>> +      *
> >>>> +      * This is called by an exporter to charge a buffer to the specified
> >>>> +      * cgroup.
> >>> Well that sentence makes absolutely no sense at all.
> >>>
> >>> The dma_buf_ops are supposed to be called by the DMA-buf subsystem on
> >>> behalves of the importer and never by the exporter itself.
> >>>
> >>> I hope that this is just a documentation mixup.
> >> Thank you for taking a look Christian!
> >>
> >> Yes, that was poor wording, sorry about that. It should instead say
> >> that the op would be called by the process the buffer is currently
> >> charged to in order to transfer the buffer's charge to a different
> >> cgroup. This is helpful in the case where a process acts as an
> >> allocator for multiple client processes and we would like the
> >> allocated buffers to be charged to the clients who requested their
> >> allocation(instead of the allocating process as is the default
> >> behavior). In Android, the graphics allocator HAL process[1] does
> >> most of the graphics allocations on behalf of various clients. After
> >> allocation, the HAL process passes the fd to the client over binder
> >> IPC and the binder driver invokes the charge_to_cgroup() DMA-BUF op to
> >> uncharge the buffer from the HAL process and charge it to the client
> >> process instead.
> >>
> >> [1]: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsource.android.com%2Fdevices%2Fgraphics%2Farch-bq-gralloc&amp;data=04%7C01%7Cchristian.koenig%40amd.com%7C838d25da974d4ea4257508d9db63eb70%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637782044488604857%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&amp;sdata=Qn7JeyF5Rq9tnrGw1KgNuQkpu5RbcrvPhDOa1OBJ6TU%3D&amp;reserved=0
> > For that use-case, do we really need to have the vfunc abstraction and
> > force all exporters to do something reasonable with it?
>
> I was about to write up a similar answer, but more from the technical side.
>
> Why in the world should that be done on the DMA-buf object as a
> communication function between importer and exporter?
>
> That design makes absolutely no sense at all to me.
>
> Regards,
> Christian.
>
> >
> > I think just storing the cgrpus gpu memory bucket this is charged against
> > and doing this in a generic way would be a lot better.
> >
> > That way we can also easily add other neat features in the future, like
> > e.g. ttm could take care of charge-assignement automatically maybe, or we
> > could print the current cgroups charge relationship in the sysfs info
> > file. Or anything else really.

Thank you for the comments Daniel and Christian! I made the
charge/uncharge/transfer part of the exporter implementation since it
provided exporters a choice on whether they wanted to enable cgroup
memory accounting for the buffers they were exporting. I also see the
benefits of making the charge/uncharge/transfer generic by moving it
to the DMA-BUF framework like you are suggesting though. We will move
to a more generic design in the next version of the RFC.

Regards,
Hridya

> >
> > I do feel that in general for gpu memory cgroups to be useful, we should
> > really have memory pools as a fairly strong concept. Otherwise every
> > driver/allocator/thing is going to come up with their own, and very likely
> > incompatible interpretation. And we end up with a supposed generic cgroups
> > interface which cannot actually be used in a driver/vendor agnostic way at
> > all.
> > -Daniel
> >
> >> Regards,
> >> Hridya
> >>
> >>
> >>> Regards,
> >>> Christian.
> >>>
> >>>>    The caller must hold a reference to @gpucg obtained via
> >>>> +      * gpucg_get(). The DMA-BUF will be uncharged from the cgroup it is
> >>>> +      * currently charged to before being charged to @gpucg. The caller must
> >>>> +      * belong to the cgroup the buffer is currently charged to.
> >>>> +      *
> >>>> +      * This callback is optional.
> >>>> +      *
> >>>> +      * Returns:
> >>>> +      *
> >>>> +      * 0 on success or negative error code on failure.
> >>>> +      */
> >>>> +     int (*charge_to_cgroup)(struct dma_buf *dmabuf, struct gpucg *gpucg);
> >>>>    };
> >>>>
> >>>>    /**
>