[Intel-gfx] [PATCH 6/6] drm/i915: Migrate stolen objects before hibernation

Tvrtko Ursulin tvrtko.ursulin at linux.intel.com
Thu Dec 10 06:15:53 PST 2015

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 at intel.com wrote:
>>> From: Chris Wilson <chris at 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?

>>> v2:
>>> Swap PTE for pinned bindings over to the shmemfs. This adds a
>>> complicated dance, but is required as many stolen objects are likely to
>>> be pinned for use by the hardware. Swapping the PTEs should not result
>>> in externally visible behaviour, as each PTE update should be atomic and
>>> the two pages identical. (danvet)
>>> safe-by-default, or the principle of least surprise. We need a new flag
>>> to mark objects that we can wilfully discard and recreate across
>>> hibernation. (danvet)
>>> Just use the global_list rather than invent a new stolen_list. This is
>>> the slowpath hibernate and so adding a new list and the associated
>>> complexity isn't worth it.
>>> v3: Rebased on drm-intel-nightly (Ankit)
>>> v4: Use insert_page to map stolen memory backed pages for migration to
>>> shmem (Chris)
>>> v5: Acquire mutex lock while copying stolen buffer objects to shmem (Chris)
>>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
>>> Signed-off-by: Ankitprasad Sharma <ankitprasad.r.sharma at intel.com>
>>> ---
>>>    drivers/gpu/drm/i915/i915_drv.c         |  17 ++-
>>>    drivers/gpu/drm/i915/i915_drv.h         |   7 +
>>>    drivers/gpu/drm/i915/i915_gem.c         | 232 ++++++++++++++++++++++++++++++--
>>>    drivers/gpu/drm/i915/intel_display.c    |   3 +
>>>    drivers/gpu/drm/i915/intel_fbdev.c      |   6 +
>>>    drivers/gpu/drm/i915/intel_pm.c         |   2 +
>>>    drivers/gpu/drm/i915/intel_ringbuffer.c |   6 +
>>>    7 files changed, 261 insertions(+), 12 deletions(-)
>>> diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
>>> index 9f55209..2bb9e9e 100644
>>> --- a/drivers/gpu/drm/i915/i915_drv.c
>>> +++ b/drivers/gpu/drm/i915/i915_drv.c
>>> @@ -1036,6 +1036,21 @@ static int i915_pm_suspend(struct device *dev)
>>>    	return i915_drm_suspend(drm_dev);
>>>    }
>>> +static int i915_pm_freeze(struct device *dev)
>>> +{
>>> +	int ret;
>>> +
>>> +	ret = i915_gem_freeze(pci_get_drvdata(to_pci_dev(dev)));
>>> +	if (ret)
>>> +		return ret;
>> One of the first steps in idling GEM seems to be idling the GPU and
>> retiring requests.
>> Would it also make sense to do those steps before attempting to migrate
>> the stolen objects?
> Here, we do that implicitly when trying to do a vma_unbind for the
> object.

Code paths are not the same so it makes me uncomfortable.  It looks more 
logical to do the migration after the existing i915_gem_suspend. It 
would mean some code duplication, true (maybe split i915_drm_suspend in 
two and call i915_gem_freeze in between), but to me it looks more like a 
proper place to do it.

Do Chris or Ville have any opinions here?



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