Re: [PATCH v4 11/15] drm/shmem-helper: Add generic memory shrinker

Rob Clark
2022-06-19T16:54:33Z

On Thu, Apr 28, 2022 at 11:20 AM Dmitry Osipenko
<dmitry.osipenko@collabora.com> wrote:
>
> 27.04.2022 18:03, Daniel Vetter wrote:
> >> ...
> >>>> @@ -172,6 +172,41 @@ struct drm_gem_object_funcs {
> >>>> * This is optional but necessary for mmap support.
> >>>> */
> >>>> const struct vm_operations_struct *vm_ops;
> >>>> +
> >>>> + /**
> >>>> + * @purge:
> >>>> + *
> >>>> + * Releases the GEM object's allocated backing storage to the
> >>>> system.
> >>>> + *
> >>>> + * Returns the number of pages that have been freed by purging
> >>>> the GEM object.
> >>>> + *
> >>>> + * This callback is used by the GEM shrinker.
> >>>> + */
> >>>> + unsigned long (*purge)(struct drm_gem_object *obj);
> >
> > Hm I feel like drivers shouldn't need to know the difference here?
> >
> > Like shmem helpers can track what's purgeable, and for eviction/purging
> > the driver callback should do the same?
> >
> > The only difference is when we try to re-reserve the backing storage. When
> > the object has been evicted that should suceed, but when the object is
> > purged that will fail.
> >
> > That's the difference between evict and purge for drivers?
>
> When buffer is purged, we can permanently release the backing storage
> and the reserved IOV space, re-using the freed space by new BOs.
>
> When buffer is evicted, the BO's IOV should be kept reserved and the
> re-reservation of the backing storage should succeed.
>
> >>>> +
> >>>> + /**
> >>>> + * @evict:
> >>>> + *
> >>>> + * Unpins the GEM object's allocated backing storage, allowing
> >>>> shmem pages
> >>>> + * to be swapped out.
> >>>
> >>> What's the difference to the existing unpin() callback?
> >>
> >> Drivers need to do more than just unpinning pages when GEMs are evicted.
> >> Unpinning is only a part of the eviction process. I'll improve the
> >> doc-comment in v5.
> >>
> >> For example, for VirtIO-GPU driver we need to to detach host from the
> >> guest's memory before pages are evicted [1].
> >>
> >> [1]
> >> https://gitlab.collabora.com/dmitry.osipenko/linux-kernel-rd/-/blob/932eb03198bce3a21353b09ab71e95f1c19b84c2/drivers/gpu/drm/virtio/virtgpu_object.c#L145
> >>
> >> In case of Panfrost driver, we will need to remove mappings before pages
> >> are evicted.
> >
> > It might be good to align this with ttm, otoh that all works quite a bit
> > differently for ttm since ttm supports buffer moves and a lot more fancy
> > stuff.
> >
> > I'm bringing this up since I have this fancy idea that eventually we could
> > glue shmem helpers into ttm in some cases for managing buffers when they
> > sit in system memory (as opposed to vram).
>
> I'll take a look at ttm for v6.
>
> >>>> + *
> >>>> + * Returns the number of pages that have been unpinned.
> >>>> + *
> >>>> + * This callback is used by the GEM shrinker.
> >>>> + */
> >>>> + unsigned long (*evict)(struct drm_gem_object *obj);
> >>>> +
> >>>> + /**
> >>>> + * @swap_in:
> >>>> + *
> >>>> + * Pins GEM object's allocated backing storage if it was
> >>>> previously evicted,
> >>>> + * moving swapped out pages back to memory.
> >>>> + *
> >>>> + * Returns 0 on success, or -errno on error.
> >>>> + *
> >>>> + * This callback is used by the GEM shrinker.
> >>>> + */
> >>>> + int (*swap_in)(struct drm_gem_object *obj);
> >>>
> >>> Why do you need swap_in()? This can be done on-demand as part of a pin
> >>> or vmap operation.
> >>
> >> Similarly to the unpinning, the pining of pages is only a part of what
> >> needs to be done for GPU drivers. Besides of returning pages back to
> >> memory, we also need to make them accessible to GPU and this is a
> >> driver-specific process. This why we need the additional callbacks.
> >
> > This is a bit much midlayer. The way this works in ttm is you reserve all
> > the objects you need (which makes sure they're physically available
> > again), and then the driver goes through and makes sure the page tables
> > are all set up again.
> >
> > Once you get towards gpu vm that's really the only approach, since your
> > swap_in has no idea for which vm it needs to restore pagetables (and
> > restoring it for all is a bit meh).
> >
> > If drivers want to optimize this they can adjust/set any tracking
> > information from their evict callback as needed.
>
> In practice, majority of BOs have only one mapping. Only shared BOs
> usually have extra mappings and shared BOs aren't evictable.
>
> When memory pages are gone, then all the GPU mappings also should be
> gone. Perhaps it's indeed won't be a bad idea to move out the restoring
> of h/w VMs from the swap_in() and make drivers to handle the restoring
> by themselves, so swap_in() will be only about restoring the pages. I'll
> try to improve it in v6.
>
> >>>> };
> >>>> /**
> >>>> diff --git a/include/drm/drm_gem_shmem_helper.h
> >>>> b/include/drm/drm_gem_shmem_helper.h
> >>>> index 70889533962a..a65557b446e6 100644
> >>>> --- a/include/drm/drm_gem_shmem_helper.h
> >>>> +++ b/include/drm/drm_gem_shmem_helper.h
> >>>> @@ -6,6 +6,7 @@
> >>>> #include <linux/fs.h>
> >>>> #include <linux/mm.h>
> >>>> #include <linux/mutex.h>
> >>>> +#include <linux/shrinker.h>
> >>>> #include <drm/drm_file.h>
> >>>> #include <drm/drm_gem.h>
> >>>> @@ -15,8 +16,18 @@
> >>>> struct dma_buf_attachment;
> >>>> struct drm_mode_create_dumb;
> >>>> struct drm_printer;
> >>>> +struct drm_device;
> >>>> struct sg_table;
> >>>> +enum drm_gem_shmem_pages_state {
> >>>> + DRM_GEM_SHMEM_PAGES_STATE_PURGED = -2,
> >>>> + DRM_GEM_SHMEM_PAGES_STATE_EVICTED = -1,
> >>>> + DRM_GEM_SHMEM_PAGES_STATE_UNPINNED = 0,
> >>>> + DRM_GEM_SHMEM_PAGES_STATE_PINNED = 1,
> >>>> + DRM_GEM_SHMEM_PAGES_STATE_EVICTABLE = 2,
> >>>> + DRM_GEM_SHMEM_PAGES_STATE_PURGEABLE = 3,
> >>>> +};
> >>>
> >>> These states can be detected by looking at the vmap and pin refcounts.
> >>> No need to store them explicitly.
> >>
> >> I'll try to revisit this, but I was finding that it's much more
> >> difficult to follow and debug code without the explicit states.
> >
> > purgeable/purged needs some state, but pinned shouldn't be duplicated, so
> > I concur here a bit.
> >
> >>> In your patch, they also come with a
> >>> big zoo of trivial helpers. None of that seems necessary AFAICT.
> >>
> >> There are couple functions which could be squashed, although this may
> >> hurt readability of the code a tad. I'll try to take another look at
> >> this for v5.
> >>
> >>> What's the difference between purge and evict BTW?
> >>
> >> The evicted pages are moved out from memory to a SWAP partition or file.
> >>
> >> The purged pages are destroyed permanently.
> >>
> >>>> +
> >>>> /**
> >>>> * struct drm_gem_shmem_object - GEM object backed by shmem
> >>>> */
> >>>> @@ -43,8 +54,8 @@ struct drm_gem_shmem_object {
> >>>> * @madv: State for madvise
> >>>> *
> >>>> * 0 is active/inuse.
> >>>> + * 1 is not-needed/can-be-purged
> >>>> * A negative value is the object is purged.
> >>>> - * Positive values are driver specific and not used by the helpers.
> >>>> */
> >>>> int madv;
> >>>> @@ -91,6 +102,40 @@ struct drm_gem_shmem_object {
> >>>> * @map_wc: map object write-combined (instead of using shmem
> >>>> defaults).
> >>>> */
> >>>> bool map_wc;
> >>>> +
> >>>> + /**
> >>>> + * @eviction_disable_count:
> >>>> + *
> >>>> + * The shmem pages are disallowed to be evicted by the memory
> >>>> shrinker
> >>>> + * while count is non-zero. Used internally by memory shrinker.
> >>>> + */
> >>>> + unsigned int eviction_disable_count;
> >>>> +
> >>>> + /**
> >>>> + * @purging_disable_count:
> >>>> + *
> >>>> + * The shmem pages are disallowed to be purged by the memory
> >>>> shrinker
> >>>> + * while count is non-zero. Used internally by memory shrinker.
> >>>> + */
> >>>> + unsigned int purging_disable_count;
> >
> > What are these disable counts for?
>
> Some of BO types should stay pinned permanently, this applies to both
> VirtIO and Panfrost drivers that make use of the generic shrinker in
> this patchset. Hence I made objects unpurgeable and unevictable by default.
>
> Initially the idea of these counts was to allow drivers to explicitly
> disable purging and eviction, and do it multiple times. If driver
> disables eviction in two different places in the code, then we need to
> track the eviction-disable count.
>
> In the v5 of this patchset drivers don't need to explicitly disable
> shrinking anymore, they only need to enable it. The counts are also used
> internally by DRM SHMEM core to track the vmappings and pinnings, but
> perhaps pages_use_count could be used for that instead. I'll revisit it
> for v6.
>
> > The way purgeable works in other drivers is that userspace sets purgeable
> > or not, and it's up to userspace to not make a mess of this.
> >
> > There's also some interactions, and I guess a bunch of drivers get this
> > wrong in funny ways. Not sure how to best clean this up.
> >
> > - Once you have a shrinker/dynamic memory management you should _not_ pin
> > pages, except when it's truly permanent like for scanout. Instead
> > drivers should attach dma_fence to the dma_resv to denote in-flight
> > access.
>
> By default pages are pinned when drm_gem_shmem_get_pages_sgt() is
> invoked by drivers during of BO creation time.
>
> We could declare that pages_use_count=1 means the pages are allowed to
> be evicted and purged if shrinker is enabled. Then the further
> drm_gem_shmem_pin/vmap() calls will bump the pages_use_count,
> disallowing the eviction and purging, like you're suggesting, and we
> won't need the explicit counts.
>
> > - A pinned buffer object is not allowed to be put into purgeable state,
> > and a bo in purgeable state should not be allowed to be pinned.
> >
> > - Drivers need to hold dma_resv_lock for long enough in their command
> > submission, i.e. from the point where the reserve the buffers and make
> > sure that mappings exists, to the point where the request is submitted
> > to hw or drm/sched and fences are installed.
> >
> > But I think a lot of current shmem users just pin as part of execbuf, so
> > this won't work quite so well right out of the box.
>
> The current shmem users assume that BO is pinned permanently once it has
> been created.
>
> > Anyway with that design I don't think there should ever be a need to
> > disable shrinking.
>
> To me what you described mostly matches to what I did in the v5.
>
> >>>> +
> >>>> + /**
> >>>> + * @pages_state: Current state of shmem pages. Used internally by
> >>>> + * memory shrinker.
> >>>> + */
> >>>> + enum drm_gem_shmem_pages_state pages_state;
> >>>> +
> >>>> + /**
> >>>> + * @evicted: True if shmem pages were evicted by the memory
> >>>> shrinker.
> >>>> + * Used internally by memory shrinker.
> >>>> + */
> >>>> + bool evicted;
> >>>> +
> >>>> + /**
> >>>> + * @pages_shrinkable: True if shmem pages can be evicted or purged
> >>>> + * by the memory shrinker. Used internally by memory shrinker.
> >>>> + */
> >>>> + bool pages_shrinkable;
> >>>
> >>> As commented before, this state can be foundby looking at existing
> >>> fields. No need to store it separately.
> >>
> >> When we're transitioning from "evictable" to a "purgeable" state, we
> >> must not add pages twice to the "shrinkable_count" variable. Hence this
> >> is not a state, but a variable which prevents the double accounting of
> >> the pages. Please see drm_gem_shmem_add_pages_to_shrinker() in this patch.
> >>
> >> Perhaps something like "pages_accounted_by_shrinker" could be a better
> >> name for the variable. I'll revisit this for v5.
> >
> > Hm not sure we need to account this? Usually the shrinker just counts when
> > it's asked to do so, not practively maintain that count. Once you start
> > shrinking burning cpu time is generally not too terrible.
>
> We could count pages on demand by walking up the "evictable" list, but
> then the shrinker's lock needs to be taken by the
> drm_gem_shmem_shrinker_count_objects() to protect the list.
>
> Previously Rob Clark said that the profiling of freedreno's shrinker
> showed that it's worthwhile to reduce the locks as much as possible,
> including the case of counting shrinkable objects.

Sorry I missed this earlier, but danvet is giving some bad advice here ;-)

You *really* need count_objects() to be lockless and fast, ie. no list
iteration. It doesn't have to return the "perfect" value, so it is ok
if it is racy / not-atomic / etc. Otherwise you will have bad system
performance issues when you start hitting do_shrink_slab() on many
threads at once.

BR,
-R