Bas has the problem that CS implicitly waits for VM updates.
Currently when you unmap a BO the operation will only be executed after
all the previously made CS are finished.
Similar for mapping BOs. The next CS will only start after all the
pending page table updates are completed.
The mapping case was already handled by my prototype patch set, but the
unmapping case still hurts a bit.
This implicit sync between CS and map/unmap operations can really hurt
the performance of applications which massively use PRTs.
Regards,
Christian.
Am 01.06.22 um 18:27 schrieb Marek Olšák:
Can you please summarize what this is about?
Thanks,
Marek
On Wed, Jun 1, 2022 at 8:40 AM Christian König
<[email protected]> wrote:
Hey guys,
so today Bas came up with a new requirement regarding the explicit
synchronization to VM updates and a bunch of prototype patches.
I've been thinking about that stuff for quite some time before,
but to
be honest it's one of the most trickiest parts of the driver.
So my current thinking is that we could potentially handle those
requirements like this:
1. We add some new EXPLICIT flag to context (or CS?) and VM IOCTL.
This
way we either get the new behavior for the whole CS+VM or the old
one,
but never both mixed.
2. When memory is unmapped we keep around the last unmap operation
inside the bo_va.
3. When memory is freed we add all the CS fences which could
access that
memory + the last unmap operation as BOOKKEEP fences to the BO and as
mandatory sync fence to the VM.
Memory freed either because of an eviction or because of userspace
closing the handle will be seen as a combination of unmap+free.
The result is the following semantic for userspace to avoid implicit
synchronization as much as possible:
1. When you allocate and map memory it is mandatory to either wait
for
the mapping operation to complete or to add it as dependency for
your CS.
If this isn't followed the application will run into CS faults
(that's what we pretty much already implemented).
2. When memory is freed you must unmap that memory first and then
wait
for this unmap operation to complete before freeing the memory.
If this isn't followed the kernel will add a forcefully wait
to the
next CS to block until the unmap is completed.
3. All VM operations requested by userspace will still be executed in
order, e.g. we can't run unmap + map in parallel or something like
this.
Is that something you guys can live with? As far as I can see it
should
give you the maximum freedom possible, but is still doable.
Regards,
Christian.
