linux-kernel - Re: [PATCH v19 22/30] drm/shmem-helper: Add common memory shrinker

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Date: Fri, 26 Jan 2024 10:55:37 +0100
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Dmitry Osipenko <dmitry.osipenko@...labora.com>
Cc: David Airlie <airlied@...il.com>, Gerd Hoffmann <kraxel@...hat.com>,
 Gurchetan Singh <gurchetansingh@...omium.org>, Chia-I Wu
 <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>, Christian König
 <christian.koenig@....com>, Qiang Yu <yuq825@...il.com>, Steven Price
 <steven.price@....com>, Emma Anholt <emma@...olt.net>, Melissa Wen
 <mwen@...lia.com>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, kernel@...labora.com,
 virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v19 22/30] drm/shmem-helper: Add common memory shrinker

On Fri, 26 Jan 2024 00:56:47 +0300
Dmitry Osipenko <dmitry.osipenko@...labora.com> wrote:

> On 1/25/24 13:19, Boris Brezillon wrote:
> > On Fri,  5 Jan 2024 21:46:16 +0300
> > Dmitry Osipenko <dmitry.osipenko@...labora.com> wrote:
> >   
> >> +static bool drm_gem_shmem_is_evictable(struct drm_gem_shmem_object *shmem)
> >> +{
> >> +	return (shmem->madv >= 0) && shmem->base.funcs->evict &&
> >> +		refcount_read(&shmem->pages_use_count) &&
> >> +		!refcount_read(&shmem->pages_pin_count) &&
> >> +		!shmem->base.dma_buf && !shmem->base.import_attach &&
> >> +		!shmem->evicted;  
> > 
> > Are we missing
> > 
> >                 && dma_resv_test_signaled(shmem->base.resv,
> > 					  DMA_RESV_USAGE_BOOKKEEP)
> > 
> > to make sure the GPU is done using the BO?
> > The same applies to drm_gem_shmem_is_purgeable() BTW.
> > 
> > If you don't want to do this test here, we need a way to let drivers
> > provide a custom is_{evictable,purgeable}() test.
> > 
> > I guess we should also expose drm_gem_shmem_shrinker_update_lru_locked()
> > to let drivers move the GEMs that were used most recently (those
> > referenced by a GPU job) at the end of the evictable LRU.  
> 
> We have the signaled-check in the common drm_gem_evict() helper:
> 
> https://elixir.bootlin.com/linux/v6.8-rc1/source/drivers/gpu/drm/drm_gem.c#L1496

Ah, indeed. I'll need DMA_RESV_USAGE_BOOKKEEP instead of
DMA_RESV_USAGE_READ in panthor, but I can add it in the driver specific
->evict() hook (though that means calling dma_resv_test_signaled()
twice, which is not great, oh well).

The problem about the evictable LRU remains though: we need a way to let
drivers put their BOs at the end of the list when the BO has been used
by the GPU, don't we?