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

[Christian König]

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@xxxxxxx> 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@xxxxxxxxxx>
> > > > ---
> > > >    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.
>
> 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);
> > > >    };
> > > >
> > > >    /**