On 10/12/15 14:15, Tvrtko Ursulin wrote:
On 10/12/15 13:17, Ankitprasad Sharma wrote:
On Thu, 2015-12-10 at 09:43 +0000, Tvrtko Ursulin wrote:
Hi,
Two more comments below:
On 09/12/15 12:46, ankitprasad.r.sharma@xxxxxxxxx wrote:
From: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Ville reminded us that stolen memory is not preserved across
hibernation, and a result of this was that context objects now being
allocated from stolen were being corrupted on S4 and promptly hanging
the GPU on resume.
We want to utilise stolen for as much as possible (nothing else will
use
that wasted memory otherwise), so we need a strategy for handling
general objects allocated from stolen and hibernation. A simple
solution
is to do a CPU copy through the GTT of the stolen object into a fresh
shmemfs backing store and thenceforth treat it as a normal objects.
This
can be refined in future to either use a GPU copy to avoid the slow
uncached reads (though it's hibernation!) and recreate stolen objects
upon resume/first-use. For now, a simple approach should suffice for
testing the object migration.
Mention of "testing" in the commit message and absence of a path to
migrate the objects back to stolen memory on resume makes me think this
is kind of half finished and note really ready for review / merge ?
Because I don't see how it is useful to migrate it one way and never
move back?
I think that this is not much of a problem, as the purpose here is to
keep the object intact, to avoid breaking anything.
So as far as objects are concerned they will be in shmem and can be used
without any issue, and the stolen memory will be free again for other
usage from the user.
I am not sure that is a good state of things.
One of the things it means is that when user wanted to create an object
in stolen memory, after resume it will not be any more. So what is the
point in failing stolen object creation when area is full in the first
place? We could just return a normal object instead.
Then the question of objects which are allocated in stolen by the
driver. Are they being re-allocated on resume or will also be stuck in
shmemfs from then onward?
And finally, one corner case might be that shmemfs plus stolen is a
larger sum which will be attempted to restored in shmemfs only on
resume. Will that always work if everything is fully populated and what
will happen if we run out of space?
At minimum all this should be discussed and explicitly documented in the
commit message.
Would it be difficult to implement the reverse path?
Please don't migrate random objects to stolen! It has all sorts of
limitations that make it unsuitable for some types of object (e.g.
contexts).
Only objects that were originally placed in stolen should ever be
candidates for the reverse migration ...
.Dave.
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