Re: [PATCH v3 09/14] drm/panthor: Add the heap logical block

[Boris Brezillon <boris.brezillon@collabora.com>]

On Fri, 8 Dec 2023 16:27:39 +0000
Steven Price <steven.price@arm.com> wrote:

> On 04/12/2023 17:33, Boris Brezillon wrote:
> > Tiler heap growing requires some kernel driver involvement: when the
> > tiler runs out of heap memory, it will raise an exception which is
> > either directly handled by the firmware if some free heap chunks are
> > available in the heap context, or passed back to the kernel otherwise.
> > The heap helpers will be used by the scheduler logic to allocate more
> > heap chunks to a heap context, when such a situation happens.
> > 
> > Heap context creation is explicitly requested by userspace (using
> > the TILER_HEAP_CREATE ioctl), and the returned context is attached to a
> > queue through some command stream instruction.
> > 
> > All the kernel does is keep the list of heap chunks allocated to a
> > context, so they can be freed when TILER_HEAP_DESTROY is called, or
> > extended when the FW requests a new chunk.
> > 
> > v3:
> > - Add a FIXME for the heap OOM deadlock  
> 
> Sadly I don't have any better solutions that what you've described in
> the FIXME.

Fully fixing the problem indeed requires having non-blocking/failable BO
allocation helpers (which is something we have on our TODO), but there's
something we might want to address now: the heap chunk allocation
currently happens with the scheduler lock held, which prevents the job
timeout from killing the group and leads to an actual deadlock of the
whole scheduler. I think we should deffer the heap chunk allocation to
a work, queued to the different wq (that's what kbase does BTW).