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

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

On Fri, 26 Jan 2024 19:27:49 +0300
Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:

> On 1/26/24 12:55, Boris Brezillon wrote:
> > On Fri, 26 Jan 2024 00:56:47 +0300
> > Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:
> >   
> >> On 1/25/24 13:19, Boris Brezillon wrote:  
> >>> On Fri,  5 Jan 2024 21:46:16 +0300
> >>> Dmitry Osipenko <dmitry.osipenko@collabora.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).  
> 
> Maybe we should change drm_gem_evict() to use BOOKKEEP. The
> test_signaled(BOOKKEEP) should be a "stronger" check than
> test_signaled(READ)?

It is, just wondering if some users have a good reason to want
READ here.

> 
> > 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?  
> 
> If BO is use, then it won't be evicted, while idling BOs will be
> evicted. Hence, the used BOs will be naturally moved down the LRU list
> each time shrinker is invoked.
> 

That only do the trick if the BOs being used most often are busy when
the shrinker kicks in though. Let's take this scenario:


BO 1					BO 2					shinker

					busy
					idle (first-pos-in-evictable-LRU)

busy
idle (second-pos-in-evictable-LRU)

					busy
					idle

					busy
					idle

					busy
					idle

										find a BO to evict
										pick BO 2

					busy (swapin)
					idle

If the LRU had been updated at each busy event, BO 1 should have
been picked for eviction. But we evicted the BO that was first
recorded idle instead of the one that was least recently
recorded busy.