Re: [PATCH 0/4] Let userspace explicitly trigger memory reclaims

[Steven Price <steven.price@arm.com>]

On 06/05/2026 11:45, Nicolas Frattaroli wrote:
> RAM is not, in fact, cheap. Especially on embedded systems with a low
> amount of memory, but known and well-defined userspace, more explicit
> resource management can lead to better utilisation patterns. As an
> example, a resource manager process on a purpose-built device may wish
> to launch, and then explicitly swap out, memory of processes that are
> kept "warm", to improve perceived startup latency of individual
> full-screen applications without making the kernel figure out the usage
> pattern from observation alone in order to swap out the right pages.

Have you considered memory control groups (memcg) for this purpose?
Imposing a lower limit than currently allocated should trigger reclaim,
so 'background' applications could have the limit lowered and then
restored when moved to the foreground.

> To allow for this explicit control in the context of panthor's GPU
> memory, add two new sysfs knobs. The first, mem_reclaim, runs an
> explicit priv BO reclaim cycle on the TGID written to it.
> 
> The second, mem_claim, does the opposite: it swaps BOs back into active
> memory.

How necessary is this mem_claim for performance? Have you done any
benchmarking of explicitly claiming vs just allowing it to happen
naturally? My gut feeling is that mem_claim should be unnecessary in
most situations, but I'm prepared to be proved wrong.

I'm not saying this series is necessarily the wrong approach - but I
think we need a bit more justification for adding a new API for this.

Thanks,
Steve

> Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
> ---
> Nicolas Frattaroli (4):
>       drm/panthor: Add freed_sz parameter to reclaim_priv_bos
>       MAINTAINERS: Add sysfs ABI docs to list of panthor files
>       drm/panthor: Add explicit memory reclaim sysfs knob
>       drm/panthor: Add explicit memory claim sysfs knob
> 
>  Documentation/ABI/testing/sysfs-driver-panthor-mem | 34 ++++++++
>  MAINTAINERS                                        |  1 +
>  drivers/gpu/drm/panthor/panthor_drv.c              | 93 ++++++++++++++++++++++
>  drivers/gpu/drm/panthor/panthor_gem.c              |  7 +-
>  drivers/gpu/drm/panthor/panthor_gem.h              |  1 +
>  drivers/gpu/drm/panthor/panthor_mmu.c              | 70 +++++++++++++++-
>  drivers/gpu/drm/panthor/panthor_mmu.h              |  4 +
>  7 files changed, 205 insertions(+), 5 deletions(-)
> ---
> base-commit: 2c4b906cd135bbb44855287d0d0eff0ee0b47afe
> change-id: 20260506-panthor-explicit-reclaim-3dffed028d8c
> 
> Best regards,
> --  
> Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
>