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

[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://source.android.com/devices/graphics/arch-bq-gralloc

For that use-case, do we really need to have the vfunc abstraction and
force all exporters to do something reasonable with it?

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);
> > >   };
> > >
> > >   /**
> >

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch