Re: [PATCH v7 00/10] drm/panthor: Add a GEM shrinker

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

On Wed,  1 Apr 2026 15:48:44 +0200
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+swapping
> 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.
> 
> 
> Changes in v2:
> - No fundamental changes in this v2, since the feedback I got were more
>   focused on bugs than the overall approach. Check the changelog in each
>   patch for more details.
> 
> Changes in v3:
> - Mostly fixes (see the changelog in each patch)
> 
> Changes in v4:
> - Only fixes (see the changelog in each patch)
> 
> Changes in v5:
> - Only fixes (see the changelog in each patch)
> 
> Changes in v6:
> - Fix huge fault handling and various other things reported by Adrian
>   and the review bot (see the changelog in each patch for more)
> 
> Changes in v7:
> - Fix a deadlock reported by lockdep
> - Add a commit to remove an unused field
> 
> 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 (9):
>   drm/gem: Consider GEM object reclaimable if shrinking fails
>   drm/panthor: Remove unused panthor_vm_op_ctx::map::new_vma field
>   drm/panthor: Move panthor_gems_debugfs_init() to panthor_gem.c
>   drm/panthor: Group panthor_kernel_bo_xxx() helpers
>   drm/panthor: Don't call drm_gpuvm_bo_extobj_add() if the object is
>     private
>   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

Queued to drm-misc-next.

> 
>  drivers/gpu/drm/drm_gem.c                |   10 +
>  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    | 1453 ++++++++++++++++++----
>  drivers/gpu/drm/panthor/panthor_gem.h    |  136 +-
>  drivers/gpu/drm/panthor/panthor_mmu.c    |  512 ++++++--
>  drivers/gpu/drm/panthor/panthor_mmu.h    |    8 +
>  drivers/gpu/drm/panthor/panthor_sched.c  |    9 +-
>  11 files changed, 1923 insertions(+), 339 deletions(-)
>