Hi, Oak,
On 1/4/22 16:35, Zeng, Oak wrote:
Regards,
Oak
-----Original Message-----
From: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
Sent: January 4, 2022 3:29 AM
To: Zeng, Oak <oak.zeng@xxxxxxxxx>; intel-gfx@xxxxxxxxxxxxxxxxxxxxx; dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: Auld, Matthew <matthew.auld@xxxxxxxxx>
Subject: Re: [PATCH v4 2/4] drm/i915: Use the vma resource as argument for gtt binding / unbinding
Hi, Oak.
On 1/4/22 00:08, Zeng, Oak wrote:
Regards,
Oak
Looks like your emails always start with "Regards, Oak". a misconfiguration?
My mail client (outlook) is set to automatically add a regards, when I compose new mail or reply email. Not a big problem 😊
-----Original Message-----
From: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
Sent: January 3, 2022 1:58 PM
To: Zeng, Oak <oak.zeng@xxxxxxxxx>; intel-gfx@xxxxxxxxxxxxxxxxxxxxx; dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: Auld, Matthew <matthew.auld@xxxxxxxxx>
Subject: Re: [PATCH v4 2/4] drm/i915: Use the vma resource as argument for gtt binding / unbinding
Hi, Oak.
On 1/3/22 19:17, Zeng, Oak wrote:
Regards,
Oak
-----Original Message-----
From: Intel-gfx <intel-gfx-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf Of Thomas Hellström
Sent: January 3, 2022 7:00 AM
To: intel-gfx@xxxxxxxxxxxxxxxxxxxxx; dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>; Auld, Matthew <matthew.auld@xxxxxxxxx>
Subject: [PATCH v4 2/4] drm/i915: Use the vma resource as argument for gtt binding / unbinding
When introducing asynchronous unbinding, the vma itself may no longer
be alive when the actual binding or unbinding takes place.
Can we take an extra reference counter of the vma to keep the vma alive, until the actual binding/unbinding takes place?
The point here is that that's not needed, and should be avoided.
Can you explain more why "keeping vma alive until unbinding takes place" should be avoided?
As I understand it, your series introduce asynchronized unbinding. But since vma might be no longer alive at the time of unbinding.
To overcome this difficulty, you introduce a vma resource structure and you guarantee vma resource is alive at bind/unbind time. So
you can use vma resource for the bind/unbind operation. My question is, can we achieve the asynchronized unbinding still using vma
structure by keeping vma structure alive ( by ref count it). This way the change should be much smaller (compared to this series). Why
it is harmful to keep the vma alive? Maybe you have other reasons to introduce vma resource that I don't see.
When we allow asynchronous unbinding, it's allowed to immediately rebind
the vma, possibly into the same gpu virtual address, but with different
pages. And when doing that we don't want to block waiting for the unbind
to execute.
Imagine this sequence:
1. Virtual address a1 is bound to physical page p1
2. Unbind a1 from p1, asynchronous so actual unbind not happen yet
3. bind a1 to physical page p2, page table is changed, now a1 pointing to p2 in page table.
4. #2 now take place now - since in page table, a1 points to p2 now, does a1 point to scratch page after #4?
Here, 3) will also become async, awaiting any pending unbinds in the
address range provided to 3), in particular, the bind in 3) will await
4). See i915_vma_resource_bind_dep_await(), and the discussion on
whether or not to use an interval tree for this at the start of
i915_vma_resource.c
In fact, we could allow a large number of outstanding binds
and unbinds for a vma, which makes the vma structure unsuitable to track
this, since there will no longer be a single mapping between a set of
active pages and a vma, or a virtual gpu range and a vma.
I agree that if pages - vma - virtual gpu range is not 1:1:1 mapping, we need introduce a finer-grained vma resource to for the non-1:1 mapping. I also understand the asynchronous unbinding utilize the virtual address space more effectively. But my feeling is that this non-1:1 mapping makes our program hard to understand and maintain. Since this non- 1:1 mapping is introduced by asynchronous binding/unbinding, maybe the real question here is, is it really benefit to introduce asynchronous unbinding?
That's a relevant question, which I've asked myself a couple of times.
Async unbinding has complicated things like error capture and indeed
complicates the understanding of the binding process as well.
The main gain is that we avoid a sync point at LMEM eviction, enabling
us to pipeline eviction, moving forward it may also find use in the
shrinker and for user-space prematurely wanting to re-use softpin addresses.
/Thomas
I am still not familiar enough to the codes. I suggest other experts to take a look also. @Bloomfield, Jon @Vetter, Daniel @Wilson, Chris P.
Regards,
Oak
Thanks,
/Thomas
Regards,
Oak
If the
vma is no longer alive, that means nobody uses it anymore, but the GPU
may still have work in the pipe that references the GPU virtual address.
/Thomas.
