Am 11.01.21 um 17:13 schrieb Daniel Vetter:
On Fri, Jan 08, 2021 at 04:49:55PM +0000, Grodzovsky, Andrey wrote:
Ok then, I guess I will proceed with the dummy pages list implementation then.
Andrey
________________________________
From: Koenig, Christian <Christian.Koenig@xxxxxxx>
Sent: 08 January 2021 09:52
To: Grodzovsky, Andrey <Andrey.Grodzovsky@xxxxxxx>; Daniel Vetter <daniel@xxxxxxxx>
Cc: amd-gfx@xxxxxxxxxxxxxxxxxxxxx <amd-gfx@xxxxxxxxxxxxxxxxxxxxx>; dri-devel@xxxxxxxxxxxxxxxxxxxxx <dri-devel@xxxxxxxxxxxxxxxxxxxxx>; daniel.vetter@xxxxxxxx <daniel.vetter@xxxxxxxx>; robh@xxxxxxxxxx <robh@xxxxxxxxxx>; l.stach@xxxxxxxxxxxxxx <l.stach@xxxxxxxxxxxxxx>; yuq825@xxxxxxxxx <yuq825@xxxxxxxxx>; eric@xxxxxxxxxx <eric@xxxxxxxxxx>; Deucher, Alexander <Alexander.Deucher@xxxxxxx>; gregkh@xxxxxxxxxxxxxxxxxxx <gregkh@xxxxxxxxxxxxxxxxxxx>; ppaalanen@xxxxxxxxx <ppaalanen@xxxxxxxxx>; Wentland, Harry <Harry.Wentland@xxxxxxx>
Subject: Re: [PATCH v3 01/12] drm: Add dummy page per device or GEM object
Mhm, I'm not aware of any let over pointer between TTM and GEM and we
worked quite hard on reducing the size of the amdgpu_bo, so another
extra pointer just for that corner case would suck quite a bit.
We have a ton of other pointers in struct amdgpu_bo (or any of it's lower
things) which are fairly single-use, so I'm really not much seeing the
point in making this a special case. It also means the lifetime management
becomes a bit iffy, since we can't throw away the dummy page then the last
reference to the bo is released (since we don't track it there), but only
when the last pointer to the device is released. Potentially this means a
pile of dangling pages hanging around for too long.
Yeah, all of them are already on my TODO list, but see below.
If you need some ideas for redundant pointers:
- destroy callback (kinda not cool to not have this const anyway), we
could refcount it all with the overall gem bo. Quite a bit of work.
The bigger problems is that TTM based drivers are using the destroy
callback pointer to distinct ghost objects from real ones.
We first need to get rid of those. I already have a plan for that and
~20% of it implemented, but it is more complicated because of the driver
specific backends in Nouveau, Amdgpu and vmwgfx.
- bdev pointer, if we move the device ttm stuff into struct drm_device, or
create a common struct ttm_device, we can ditch that
Yes, exactly that's what my device structure rename patch set is aiming
for :)
- We could probably merge a few of the fields and find 8 bytes somewhere
Please point out where.
- we still have 2 krefs, would probably need to fix that before we can
merge the destroy callbacks
Yes, already on my TODO list as well. But the last time I looked into
this I was blocked by the struct_mutex once more.
So there's plenty of room still, if the size of a bo struct is really that
critical. Imo it's not.
It is. See we had a size of struct amdgpu_bo of over 1500 bytes because
we stopped caring for that, no we are down to 816 at the moment.
We really need to get rid of this duplication of functionality and
structure between TTM and GEM.
Christian.
-Daniel
Christian.
Am 08.01.21 um 15:46 schrieb Andrey Grodzovsky:
Daniel had some objections to this (see bellow) and so I guess I need
you both to agree on the approach before I proceed.
Andrey
On 1/8/21 9:33 AM, Christian König wrote:
Am 08.01.21 um 15:26 schrieb Andrey Grodzovsky:
Hey Christian, just a ping.
Was there any question for me here?
As far as I can see the best approach would still be to fill the VMA
with a single dummy page and avoid pointers in the GEM object.
Christian.
Andrey
On 1/7/21 11:37 AM, Andrey Grodzovsky wrote:
On 1/7/21 11:30 AM, Daniel Vetter wrote:
On Thu, Jan 07, 2021 at 11:26:52AM -0500, Andrey Grodzovsky wrote:
On 1/7/21 11:21 AM, Daniel Vetter wrote:
On Tue, Jan 05, 2021 at 04:04:16PM -0500, Andrey Grodzovsky wrote:
On 11/23/20 3:01 AM, Christian König wrote:
Am 23.11.20 um 05:54 schrieb Andrey Grodzovsky:
On 11/21/20 9:15 AM, Christian König wrote:
Am 21.11.20 um 06:21 schrieb Andrey Grodzovsky:
Will be used to reroute CPU mapped BO's page faults once
device is removed.
Uff, one page for each exported DMA-buf? That's not
something we can do.
We need to find a different approach here.
Can't we call alloc_page() on each fault and link them together
so they are freed when the device is finally reaped?
For sure better to optimize and allocate on demand when we reach
this corner case, but why the linking ?
Shouldn't drm_prime_gem_destroy be good enough place to free ?
I want to avoid keeping the page in the GEM object.
What we can do is to allocate a page on demand for each fault
and link
the together in the bdev instead.
And when the bdev is then finally destroyed after the last
application
closed we can finally release all of them.
Christian.
Hey, started to implement this and then realized that by
allocating a page
for each fault indiscriminately
we will be allocating a new page for each faulting virtual
address within a
VA range belonging the same BO
and this is obviously too much and not the intention. Should I
instead use
let's say a hashtable with the hash
key being faulting BO address to actually keep allocating and
reusing same
dummy zero page per GEM BO
(or for that matter DRM file object address for non imported
BOs) ?
Why do we need a hashtable? All the sw structures to track this
should
still be around:
- if gem_bo->dma_buf is set the buffer is currently exported as
a dma-buf,
so defensively allocate a per-bo page
- otherwise allocate a per-file page
That exactly what we have in current implementation
Or is the idea to save the struct page * pointer? That feels a
bit like
over-optimizing stuff. Better to have a simple implementation
first and
then tune it if (and only if) any part of it becomes a problem
for normal
usage.
Exactly - the idea is to avoid adding extra pointer to
drm_gem_object,
Christian suggested to instead keep a linked list of dummy pages
to be
allocated on demand once we hit a vm_fault. I will then also
prefault the entire
VA range from vma->vm_end - vma->vm_start to vma->vm_end and map
them
to that single dummy page.
This strongly feels like premature optimization. If you're worried
about
the overhead on amdgpu, pay down the debt by removing one of the
redundant
pointers between gem and ttm bo structs (I think we still have
some) :-)
Until we've nuked these easy&obvious ones we shouldn't play "avoid 1
pointer just because" games with hashtables.
-Daniel
Well, if you and Christian can agree on this approach and suggest
maybe what pointer is
redundant and can be removed from GEM struct so we can use the
'credit' to add the dummy page
to GEM I will be happy to follow through.
P.S Hash table is off the table anyway and we are talking only
about linked list here since by prefaulting
the entire VA range for a vmf->vma i will be avoiding redundant
page faults to same VMA VA range and so
don't need to search and reuse an existing dummy page but simply
create a new one for each next fault.
Andrey
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