Re: [PATCH v1 0/9] drm/panthor: Add a GEM shrinker

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

+Chia-I

On Fri,  9 Jan 2026 14:07:52 +0100
Boris Brezillon <boris.brezillon@collabora.com> wrote:

> Hello,
> 
> This is an attempt at adding a GEM shrinker to panthor so the system
> can finally reclaim GPU memory.
> 
> This implementation is losely based on the MSM shrinker (which is why
> I added the MSM maintainers in Cc), and it's relying on the drm_gpuvm
> eviction/validation infrastructure.
> 
> I've only done very basic IGT-based [1] and chromium-based (opening
> a lot of tabs on Aquarium until the system starts reclaiming+swaping
> out GPU buffers) testing, but I'm posting this early so I can get
> preliminary feedback on the implementation. If someone knows about
> better tools/ways to test the shrinker, please let me know.
> 
> A few words about some design/implementation choices:
> - No MADVISE support because I want to see if we can live with just
>   transparent reclaim
> - We considered basing this implementation on the generic shrinker work
>   started by Dmitry [2], but
>   1. with the activeness/idleness tracking happening at the VM
>      granularity, having per-BO LRUs would caused a lot of
>      list_move()s that are not really needed (the VM as a whole
>      become active/idle, we can track individual BOs)
>   2. Thomas Zimmermann recently suggested that we should have our
>      own GEM implementation instead of trying to add this extra reclaim
>      complexity to gem-shmem. There are some plans to create a
>      gem-uma (Unified Memory Architecture) lib that would do more
>      than gem-shmem but in a way that doesn't force all its users
>      to pay the overhead (size overhead of the gem object, mostly)
>      for features they don't use. Patch "Part ways with
>      drm_gem_shmem_object" is showing what this component-based lib
>      API could look like if it were to be extracted
> - At the moment we only support swapout, but we could add an
>   extra flag to specify when buffer content doesn't need to be
>   preserved to avoid the swapout/swapin dance. First candidate for
>   this DISCARD_ON_RECLAIM flag would probably be the tiler heap chunks.
> - Reclaim uses _try_lock() all the way because of the various lock order
>   inversions between the reclaim path and submission paths. That means
>   we don't try very hard to reclaim hot GPU buffers, but the locking is
>   such a mess that I don't really see a better option to be honest.
> 
> Regards,
> 
> Boris
> 
> [1]https://gitlab.freedesktop.org/bbrezillon/igt-gpu-tools/-/commit/fc76934a5579767d2aabe787d40e38a17c3f4ea4
> [2]https://lkml.org/lkml/2024/1/5/665
> 
> Akash Goel (1):
>   drm/panthor: Add a GEM shrinker
> 
> Boris Brezillon (8):
>   drm/gem: Consider GEM object reclaimable if shrinking fails
>   drm/gpuvm: Validate BOs in the extobj list when VM is resv protected
>   drm/panthor: Move panthor_gems_debugfs_init() to panthor_gem.c
>   drm/panthor: Group panthor_kernel_bo_xxx() helpers
>   drm/panthor: Part ways with drm_gem_shmem_object
>   drm/panthor: Lazily allocate pages on mmap()
>   drm/panthor: Split panthor_vm_prepare_map_op_ctx() to prepare for
>     reclaim
>   drm/panthor: Track the number of mmap on a BO
> 
>  drivers/gpu/drm/drm_gem.c                |   10 +
>  drivers/gpu/drm/drm_gpuvm.c              |   23 +-
>  drivers/gpu/drm/panthor/Kconfig          |    1 -
>  drivers/gpu/drm/panthor/panthor_device.c |   11 +-
>  drivers/gpu/drm/panthor/panthor_device.h |   73 ++
>  drivers/gpu/drm/panthor/panthor_drv.c    |   33 +-
>  drivers/gpu/drm/panthor/panthor_fw.c     |   16 +-
>  drivers/gpu/drm/panthor/panthor_gem.c    | 1387 ++++++++++++++++++----
>  drivers/gpu/drm/panthor/panthor_gem.h    |  135 ++-
>  drivers/gpu/drm/panthor/panthor_mmu.c    |  451 +++++--
>  drivers/gpu/drm/panthor/panthor_mmu.h    |    8 +
>  drivers/gpu/drm/panthor/panthor_sched.c  |    9 +-
>  include/drm/drm_gpuvm.h                  |    6 +
>  13 files changed, 1829 insertions(+), 334 deletions(-)
>