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

[Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>]

On Thu, 2015-12-10 at 18:00 +0000, Dave Gordon wrote:
> 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 ...
Yes, obviously. We will consider only those objects which were
originally placed in stolen area.
> 
> .Dave.
> 
Thanks,
Ankit



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