Hi,
On 7/22/21 11:59 AM, Matthew Auld wrote:
On Thu, 22 Jul 2021 at 10:49, Matthew Auld
<matthew.william.auld@xxxxxxxxx> wrote:
On Wed, 21 Jul 2021 at 21:11, Jason Ekstrand <jason@xxxxxxxxxxxxxx> wrote:
On Mon, Jul 19, 2021 at 8:35 AM Matthew Auld
<matthew.william.auld@xxxxxxxxx> wrote:
On Fri, 16 Jul 2021 at 20:49, Jason Ekstrand <jason@xxxxxxxxxxxxxx> wrote:
On Fri, Jul 16, 2021 at 1:45 PM Matthew Auld
<matthew.william.auld@xxxxxxxxx> wrote:
On Fri, 16 Jul 2021 at 18:39, Jason Ekstrand <jason@xxxxxxxxxxxxxx> wrote:
On Fri, Jul 16, 2021 at 11:00 AM Matthew Auld
<matthew.william.auld@xxxxxxxxx> wrote:
On Fri, 16 Jul 2021 at 16:52, Matthew Auld
<matthew.william.auld@xxxxxxxxx> wrote:
On Fri, 16 Jul 2021 at 15:10, Jason Ekstrand <jason@xxxxxxxxxxxxxx> wrote:
On Fri, Jul 16, 2021 at 8:54 AM Matthew Auld
<matthew.william.auld@xxxxxxxxx> wrote:
On Thu, 15 Jul 2021 at 23:39, Jason Ekstrand <jason@xxxxxxxxxxxxxx> wrote:
Whenever we had a user object (n_placements > 0), we were ignoring
obj->mm.region and always putting obj->placements[0] as the requested
region. For LMEM+SMEM objects, this was causing them to get shoved into
LMEM on every i915_ttm_get_pages() even when SMEM was requested by, say,
i915_gem_object_migrate().
i915_ttm_migrate calls i915_ttm_place_from_region() directly with the
requested region, so there shouldn't be an issue with migration right?
Do you have some more details?
With i915_ttm_migrate directly, no. But, in the last patch in the
series, we're trying to migrate LMEM+SMEM buffers into SMEM on
attach() and pin it there. This blows up in a very unexpected (IMO)
way. The flow goes something like this:
- Client attempts a dma-buf import from another device
- In attach() we call i915_gem_object_migrate() which calls
i915_ttm_migrate() which migrates as requested.
- Once the migration is complete, we call i915_gem_object_pin_pages()
which calls i915_ttm_get_pages() which depends on
i915_ttm_placement_from_obj() and so migrates it right back to LMEM.
The mm.pages must be NULL here, otherwise it would just increment the
pages_pin_count?
Given that the test is using the ____four_underscores version, it
doesn't have that check. However, this executes after we've done the
dma-buf import which pinned pages. So we should definitely have
pages.
We shouldn't call ____four_underscores() if we might already have
pages though. Under non-TTM that would leak the pages, and in TTM we
might hit the WARN_ON(mm->pages) in __i915_ttm_get_pages(), if for
example nothing was moved. I take it we can't just call pin_pages()?
Four scary underscores usually means "don't call this in normal code".
I've switched the ____four_underscores call to a __two_underscores in
the selftests and it had no effect, good or bad. But, still, probably
better to call that one.
Maybe the problem here is actually that our TTM code isn't respecting
obj->mm.pages_pin_count?
I think if the resource is moved, we always nuke the mm.pages after
being notified of the move. Also TTM is also not allowed to move
pinned buffers.
I guess if we are evicted/swapped, so assuming we are not holding the
object lock, and it's not pinned, the future call to get_pages() will
see mm.pages = NULL, even though the ttm_resource is still there, and
because we prioritise the placements[0], instead of mm.region we end
up moving it for no good reason. But in your case you are holding the
lock, or it's pinned? Also is this just with the selftest, or
something real?
Or at least in the selftest I see ____i915_gem_object_get_pages()
which doesn't even consider the mm.pages AFAIK.
The bogus migration is happening as part of the
__i915_gem_object_get_pages() (2 __underscores) call in
i915_gem_dmabuf_attach (see last patch). That code is attempting to
migrate the BO to SMEM and then pin it there using the obvious calls
to do so. However, in the pin_pages call, it gets implicitly migrated
back to LMEM thanks to i915_ttm_get_pages(). Why is _get_pages()
migrating things at all?
Not sure yet, but __two_underscores() checks if
i915_gem_object_has_pages() before actually calling into
i915_ttm_get_pages(), so the mm.pages would have to be NULL here for
some reason, so best guess is something to do with move_notify().
Did a bit of experimenting along those lines and added the following
to the self-test BEFORE the export/import:
i915_gem_object_lock(obj, NULL);
err = __i915_gem_object_get_pages(obj);
__i915_gem_object_unpin_pages(obj);
i915_gem_object_unlock(obj);
if (err) {
pr_err("__i915_gem_object_get_pages failed with err=%d\n", err);
goto out_ret;
}
This seems to make the migration happen as expected without this
patch. So it seems the problem only exists on buffers that haven't
gotten any backing storage yet (if I'm understanding get_pages
correctly).
One potential work-around (not sure if this is a good idea or not!)
would be to do this inside dmabuf_attach(). Is this reliable? Once
it has pages will it always have pages? Or are there crazy races I
need to be worried about here?
It turns out that the i915_ttm_adjust_gem_after_move() call in
ttm_object_init will always update the mm.region to system memory(so
that it matches the ttm resource), which seems reasonable given the
default system placeholder thing, but does seem slightly iffy since we
haven't actually moved/allocated anything.
So effectively i915_ttm_migrate(SYSTEM) becomes a noop here since
mm.region == mr. Which ofc means when we actually call get_pages() all
that happens is that we allocate the pages in system memory(or without
this patch placements[0]). Also with this patch lmem+smem, will always
be placed in smem first, regardless of the placements ordering.
For now we could maybe just split i915_ttm_adjust_gem_after_move() so
we skip the part which updates the mm.region here in the init portion,
since that should only happen when we try to place the object for
real?
Doesn't that mean we would end up with obj->mm.region and
obj->mm.res->mem_type are out-of-sync? That seems bad. I would think
we'd want the two in sync at all times.
It likely doesn't matter since all roads lead to i915_ttm_get_pages()
when we need to actually use the object?
Also updating the mm.region in ttm_object_init() to reflect the dummy
ttm resource seems a little scary, since any existing is_lmem() check
now needs to happen after we place the object. Or at least the
existing callers(for kernel internal objects) might not have expected
that behaviour. Not sure if we checked all the callers.
It seems like the fundamental problem here is that, when it's created,
the object isn't really in any memory region at all. While I don't
think obj->mm.region == NULL is allowed or a good idea, it does seem
closer to the ground truth.
Yeah, seems reasonable, especially for create_user where we don't know
the placement until we actually call get_pages(). I think for internal
users like with create_lmem() setting the mm.region early still makes
some sense?
Perhaps what we really want is for i915_gem_object_migrate to
get_pages before it does the migration to ensure that pages exist.
The only call to i915_gem_object_migrate in the code-base today is in
the display code and it's immediately followed by pin_pages(). For
that matter, maybe the call we actually want is
i915_object_migrate_and_pin that does the whole lot.
I guess the only downside is that we might end up doing a real
migration, with mempy or the blitter vs just changing the preferred
placement for later? I think just go with whatever you feel is the
simplest for now.
Another cheapo could be to drop the mr == mm.region noop, and just try
to place the object at mr anyway?
There are a number of things to consider here,
First, as Jason found out what's keeping thing from working as intended
is that we actually call into TTM get_pages() after migration, since the
object isn't populated with pages yet. That's indeed a bug.
We should probably have migrate be migrate_and_populate(): Whatever
kernel code decides to migrate needs to hold the object lock over the
operation where data needs to be migrated or in the worst case call
pin() under the lock which currently needs to be the case for dma-buf
and display.
If we blindly just look at obj->mm.region() in get_pages() then if an
object with allowable placements in lmem and smem initially gets placed
in lmem, and then evicted to smem it will never migrate back to lmem
unless if there is an explicit i915_gem_object_migrate(), but again,
that's perhaps what we want? I guess we need to more clearly define the
migration policies; for example should we attempt to migrate evicted
buffers back to lmem on each execbuf where they are referenced, even if
they haven't lost their pages?
On region dicrepance between gem and TTM there is a short DOC: section
in i915_gem_ttm.c
/Thomas
Thoughts?
--Jason
P.S. I'm going to go ahead and send another version with your other
comments addressed. We can keep this discussion going here for now.
