Re: [PATCH drm-next v2 05/16] drm: manager to keep track of GPUs VA mappings

From: Danilo Krummrich
Date: Mon Mar 13 2023 - 19:46:59 EST

On 3/7/23 23:43, Liam R. Howlett wrote:
* Danilo Krummrich <dakr@xxxxxxxxxx> [230306 10:46]:
On 3/2/23 03:38, Liam R. Howlett wrote:
* Danilo Krummrich <dakr@xxxxxxxxxx> [230227 08:17]:

...
Would this variant be significantly more efficient?

Well, what you are doing is walking the tree to see if there's anything
there... then re-walking the tree to store it. So, yes, it's much more
efficient.. However, writing is heavier. How much of the time is spent
walking vs writing depends on the size of the tree, but it's rather easy
to do this in a single walk of the tree so why wouldn't you?

I will, I was just curious about how much of an impact it has.



Also, would this also work while already walking the tree?

Yes, to an extent. If you are at the correct location in the tree, you
can write to that location. If you are not in the correct location and
try to write to the tree then things will go poorly.. In this scenario,
we are very much walking the tree and writing to it in two steps.


To remove an entry while walking the tree I have a separate function
drm_gpuva_iter_remove(). Would I need something similar for inserting
entries?

I saw that. Your remove function uses the erase operation which is
implemented as a walk to that location and a store of a null over the
range that is returned. You do not need a function to insert an entry
if the maple state is at the correct location, and that doesn't just
mean setting mas.index/mas.last to the correct value. There is a node &
offset saved in the maple state that needs to be in the correct
location. If you store to that node then the node may be replaced, so
other iterators that you have may become stale, but the one you used
execute the store operation will now point to the new node with the new
entry.


I already provided this example in a separate mail thread, but it may makes
sense to move this to the mailing list:

In __drm_gpuva_sm_map() we're iterating a given range of the tree, where the
given range is the size of the newly requested mapping. __drm_gpuva_sm_map()
invokes a callback for each sub-operation that needs to be taken in order to
fulfill this mapping request. In most cases such a callback just creates a
drm_gpuva_op object and stores it in a list.

However, drivers can also implement the callback, such that they directly
execute this operation within the callback.

Let's have a look at the following example:

0 a 2
old: |-----------| (bo_offset=n)

1 b 3
req: |-----------| (bo_offset=m)

0 a' 1 b 3
new: |-----|-----------| (a.bo_offset=n,b.bo_offset=m)

This would result in the following operations.

__drm_gpuva_sm_map() finds entry "a" and calls back into the driver
suggesting to re-map "a" with the new size. The driver removes entry "a"
from the tree and adds "a'"

What you have here won't work. The driver will cause your iterators
maple state to point to memory that is freed. You will either need to
pass through your iterator so that the modifications can occur with that
maple state so it remains valid, or you will need to invalidate the
iterator on every modification by the driver.

I'm sure the first idea you have will be to invalidate the iterator, but
that is probably not the way to proceed. Even ignoring the unclear
locking of two maple states trying to modify the tree, this is rather
inefficient - each invalidation means a re-walk of the tree. You may as
well not use an iterator in this case.

Depending on how/when the lookups occur, you could still iterate over
the tree and let the driver modify the ending of "a", but leave the tree
alone and just store b over whatever - but the failure scenarios may
cause you grief.

If you pass the iterator through, then you can just use it to do your
writes and keep iterating as if nothing changed.

Passing through the iterater clearly seems to be the way to go.

I assume that if the entry to insert isn't at the location of the iterator
(as in the following example) we can just keep walking to this location my
changing the index of the mas and calling mas_walk()?

no. You have to mas_set() to the value and walk from the top of the
tree. mas_walk() walks down, not from side to side - well, it does go
forward within a node (increasing offset), but if you hit the node limit
then you have gotten yourself in trouble.

This would also imply
that the "outer" tree walk continues after the entry we just inserted,
right?

I don't understand the "outer" tree walk statement.

I think I could have phrased this better. I just mean "my" iterator walking
each tree entry rather than an internal tree walk, as it happens in e.g.
mas_walk() or mas_find().



1 a 3
old: |-----------| (bo_offset=n)

0 b 2
req: |-----------| (bo_offset=m)

0 b 2 a' 3
new: |-----------|-----| (b.bo_offset=m,a.bo_offset=n+2)

Again, after finding "a", we want to remove it and insert "a'" instead.

Ah, so you could walk to 0, see that it's NULL from 0 - 1, call
mas_next() and get "a" from 1 - 3, write "a'" from 2 - 3:

0 1 a 2 a' 3
broken: |-----|------|-----| (a is broken in this 1/2 step)

mas_set_range(&mas, 0, 2); /* Resets the tree location to MAS_START */
mas_store(&mas, b);
0 b 2 a' 3
new: |-----------|-----| (b.bo_offset=m,a.bo_offset=n+2)


You can *probably* also get away with this:

walk to 0, see that it's NULL from 0 - 1, call mas_next() and get "a"
from 1 - 3, write "a'" from 2 - 3:

0 1 a 2 a' 3
broken: |-----|------|-----| (a is broken in this 1/2 step)

mas_prev(&mas, 0); /* Looking at broken a from 1-2.
mas_store(&mas, NULL); /* NULL is expanded on write to 0-2.
0 NULL 2 a' 3
broken': |-----------|-----| (b.bo_offset=m,a.bo_offset=n+2)

mas_store(&mas, b);
0 b 2 a' 3
new: |-----------|-----| (b.bo_offset=m,a.bo_offset=n+2)

You may want to iterate backwards and do the writes as you go until you
have enough room.. it really depends how you want to go about doing
things.

I see, again thanks for explaining.

I think I would prefer to either (1) have generic insert() function with a
similar behavior as when iterating through a list or (2) have a function
dedicated to the "split" use case.

1) When iterating the tree inserting entries at arbitrary locations should
not influence the next iteration step. Unless the new entry really is the
next entry, but that'd be optional. I don't see a use case for that.

2) Similar to how you broke it down above I could imagine a function
dedicated to the split operation. This would be similar to what you mention
for mmap below. However, it wouldn't be a single operation.

The GPUVA manager provides sub-operations to the driver for a single mapping
request. Those can be an arbitrary amount of unmaps (for mappings "in the
way", as you say below), one or two remaps (for splits at the beginning or
end or both) and exactly one map (which is the last sub-operation adding the
newly requested mapping).

Remaps consist out of the mapping to unmap and one or two new mappings to
map. The only case where a remap sub-op has two new mappings to map is when
the newly requested mapping is enclosed by a single existing mapping. If we
overlap a mapping at the beginning and another one at the end this would be
two separate remap sub-ops. Of course, between the two remaps there could be
an arbitrary amount of unmap sub-ops.

Unmap sub-ops are simple, I just need to remove a single entry in the tree.
drm_gpuva_iter_remove() should be fine for that.

For remap sub-ops, I would need a function that removes an entry and then
adds one or two new entries within the range of the removed one. The next
loop iteration should then continue at the entry (is any) after the range of
the removed one.

However, I'm unsure how to implement this. Would I need to just do a
mas_store() of the new entry/entries (since the nodes should already be
allocated) and then clean up the nodes that are left with mas_erase()?

Let's say there is an entry A = [0 - 5] and I want to replace it with B = [0
- 1] and C = [4 - 5].

Could I just store B and C and then somehow clean up the range [2 - 3]?

The most efficient way:
mas_set(&mas, 0);
// Walk down to 0
mas_walk(&mas);
// We are now pointing at A (index = 0, last = 5)
mas.last = 1;
// No walk here.
mas_store(&mas, B);
// Going to the next entry is very fast.
mas_next(&mas)
// We are now pointing at a fragment of A (index = 2, last = 5)
mas.last = 3;
// No walk here.
mas_store(&mas, NULL);
// Going to the next entry is very fast
mas_next(&mas);
// We are now pointing at a fragment of A (index = 4, last = 5)
mas_store(&mas, C);

Less efficient, but still fine:
// Walk down to 0 and store
mas_set_range(&mas, 0, 1);
mas_store(&mas, B);
// Reset to the top of the tree
mas_set_range(&mas, 4, 5);
// Walk down to 4 and store
mas_store(&mas, C);
// Reset to the top of the tree
mas_set_range(&mas, 2, 3);
// Walk down to 2 and store
mas_store(&mas, NULL);



Maybe 1) would be the most flexible way, however, if 2) can be implemented
more efficiently that's perfectly fine too.

You can do anything you want, but the more you can use the same maple
state and save walking from the top the more efficient it will be.
Every level is another dereference down the tree.. We do have a
branching factor of 16 here, so I don't know the size of your tree and
how worth the effort it is for you.

I think it could be worth taking the first approach and providing functions that are tied specifically to the use cases of the GPUVA manager, rather than generalizing them too much and re-walk the tree more than necessary. I think the size of the tree can be up to a couple 100k.

Since some operations may be executed from dma-fence signalling critical sections I have a use case for mas_preallocate(). I was wondering if I can ignore the "entry" argument of mas_preallocate() and just pass NULL, since it's actually never used. What's the purpose of this argument? Or is it bug?







__drm_gpuva_sm_map(), ideally, continues the loop searching for nodes
starting from the end of "a" (which is 2) till the end of the requested
mapping "b" (which is 3). Since it doesn't find any other mapping within
this range it calls back into the driver suggesting to finally map "b".

If there would have been another mapping between 2 and 3 it would have
called back into the driver asking to unmap this mapping beforehand.

So, it boils down to re-mapping as described at the beginning (and
analogously at the end) of a new mapping range and removing of entries that
are enclosed by the new mapping range.

I assume the unmapped area is no longer needed, and the 're-map' is
really a removal of information? Otherwise I'd suggest searching for a
gap which fits your request. What you have here is a lot like
"MAP_FIXED" vs top-down/bottom-up search in the VMA code, this seems to
be like your __drm_gpuva_sm_map() and the drm mm range allocator with
DRM_MM_INSERT_LOW, and DRM_MM_INSERT_HIGH.

Why can these split/unmappings fail? Is it because they are still
needed?


You mean the check before the mas_*() operations in drm_gpuva_insert()?

Yes, the callbacks.


Removing entries should never fail, inserting entries should fail when the
caller tries to store to an area outside of the VA space (it doesn't
necessarily span the whole 64-bit space), a kernel reserved area of the VA
space, is not in any pre-allocated range of the VA space (if regions are
enabled) or an entry already exists at that location.

In the mmap code, I have to deal with splitting the start/end VMA and
removing any VMAs in the way. I do this by making a 'detached' tree
that is dealt with later, then just overwriting the area with one
mas_store() operation. Would something like that work for you?

I think this is pretty much the same thing I want to do, hence this should
work. However, this would require more state keeping for the whole
iteration, I guess. Drivers shouldn't know how the GPUVA manager keeps track
of mappings internally (and hence they shouldn't know about the maple tree).
If I could get away with something similar to what I wrote above, I think
I'd probably not add this extra complexity, unless there are relevant
performance reasons to do so.


Well maybe you can tell your drivers that there's something in the way
and they can remove it from their end but not alter the tree. Sort of
like a "Untracked" callback.

If you have a "This range has changed to X-Y" then you can use this
along with the "Untracked" to implement the above in a single
write. Iterate through the area , call back to the driver to alter the
start range, then keep "Untracing" until you alter the end range.

...

Cheers,
Liam