[Intel-gfx] [PATCH] drm/i915: Allow objects to go back above 4GB in the address range

[Daniel Vetter]

On Mon, Dec 14, 2015 at 10:18:02AM +0000, Michel Thierry wrote:
> On 12/11/2015 6:57 PM, Daniel Vetter wrote:
> >On Fri, Dec 11, 2015 at 02:49:52PM +0000, Chris Wilson wrote:
> >>On Fri, Dec 11, 2015 at 02:34:13PM +0000, Michel Thierry wrote:
> >>>We detected if objects should be moved to the lower parts when 48-bit
> >>>support flag was not set, but not the other way around.
> >>>
> >>>This handles the case in which an object was allocated in the 32-bit
> >>>address range, but it has been marked as safe to move above it, which
> >>>theoretically would help to keep the lower addresses available for
> >>>objects which really need to be there.
> >>>
> >>>Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
> >>>Signed-off-by: Michel Thierry <michel.thierry at intel.com>
> >>
> >>No. This is not lazy. When we run out of low space, we evict. Until then
> >>don't cause extra work for no reason.
> >
> >Yeah, this stuff should just work. First the eviction code should kick
> >stuff out, and if we totally deadlock then we'll retry with everything
> >placed nicely. Long-term objects should segregate (assuming you're not
> >mixing them up badly in the userspace cache).
> >
> >How did this come up? I think there's a more in-depth story to be shared
> >here, with some perf data to illustrate it ...
> 
> Hi,
> 
> It came from some local testing; Daniele saw bo's with the support flag
> enabled staying in the 32-bit range (the test was changing the flag between
> submissions).
> 
> If there's no space constraints, re-enabling the flag won't relocate them,
> only when evict is required as you said... and that's not really an issue.

Hm, it might make sense for validation to catch userspace bugs where it
mislabels bos wrt 48bit. But then unit tests (at least with mesa/piglit)
destroy the gpu context (well entire executable) for every test, so false
sharing like this shouldn't ever happen and we should be able to catch
issues reliably. So I think we don't need some special debugfs tunable to
kick the kernel into validation mode for this case.

And if you absolutely want it we already have the evict interface to
forcefully throw out everything and provoke fresh bindings. Not cheap, but
should still work for at least validating the logic.

> Sorry for the noise.

No worries, just figured there's a bigger story worth sharing here.
Validation noticing this makes sense.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch