Re: [PATCH 6/6] drm/i915: Migrate stolen objects before hibernation

[Dave Gordon <david.s.gordon@intel.com>]

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@intel.com wrote:
>>>> From: Chris Wilson <chris@chris-wilson.co.uk>
>>>>
>>>> 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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