On Fri, 26 Jan 2024 19:27:49 +0300
Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
> On 1/26/24 12:55, Boris Brezillon wrote:
> > On Fri, 26 Jan 2024 00:56:47 +0300
> > Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
> >
> >> On 1/25/24 13:19, Boris Brezillon wrote:
> >>> On Fri, 5 Jan 2024 21:46:16 +0300
> >>> Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> 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.
