Re: [PATCH 4/5] drm/amdgpu: Set/clear CPU_ACCESS_REQUIRED flag on page fault and CS

[John Brooks <john@fastquake.com>]

On Thu, Jun 29, 2017 at 11:35:53AM +0900, Michel Dänzer wrote:
> On 29/06/17 08:26 AM, John Brooks wrote:
> > On Wed, Jun 28, 2017 at 03:05:32PM +0200, Christian König wrote:
> >> Am 28.06.2017 um 04:33 schrieb John Brooks:
> >>> When the AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED flag is given by userspace,
> >>> it should only be treated as a hint to initially place a BO somewhere CPU
> >>> accessible, rather than having a permanent effect on BO placement.
> >>>
> >>> Instead of the flag being set in stone at BO creation, set the flag when a
> >>> page fault occurs so that it goes somewhere CPU-visible, and clear it when
> >>> the BO is requested by the GPU.
> >>>
> >>> However, clearing the CPU_ACCESS_REQUIRED flag may move BOs in GTT to
> >>> invisible VRAM, where they may promptly generate another page fault. When
> >>> BOs are constantly moved back and forth like this, it is highly detrimental
> >>> to performance. Only clear the flag on CS if:
> >>>
> >>> - The BO wasn't page faulted for a certain amount of time (currently 10
> >>>   seconds), and
> >>> - its last page fault didn't occur too soon (currently 500ms) after its
> >>>   last CS request, or vice versa.
> >>>
> >>> Setting the flag in amdgpu_fault_reserve_notify() also means that we can
> >>> remove the loop to restrict lpfn to the end of visible VRAM, because
> >>> amdgpu_ttm_placement_init() will do it for us.
> >>
> >> I'm fine with the general approach, but I'm still absolutely not keen about
> >> clearing the flag when userspace has originally specified it.
> 
> Is there any specific concern you have about that?
> 
> 
> >> Please add a new flag something like AMDGPU_GEM_CREATE_CPU_ACCESS_HINT or
> >> something like this.
> > 
> > Is it the fact that we clear a flag that userspace expects not to have changed
> > if it queries it later? I think that's the only effect of this that's directly
> > visible to userspace code.
> 
> I don't see any way for userspace to query that.

I was looking at amdgpu_gem_metadata_ioctl(). It looked like it was possible to
query a BO's flags through that method. I don't know if it actually matters; it
was just a stab in the dark for any possibly tangible effect on userspace that
might arise from the kernel changing the flag.

John

> 
> 
> > As for a new "hint" flag, I assume this new flag would be an alternative to the
> > existing CPU_ACCESS_REQUIRED flag, and we'd change Mesa et al to use it in
> > situations where the kernel *should* place a BO somewhere CPU-accessible, but
> > is permitted to move it elsewhere. Is that correct?
> 
> That seems silly. The userspace flag could never be more than a hint.
> Unfortunately we named it to suggest differently, but we have to live
> with that.
> 
> If we do need a new hint flag internally in the driver, we should simply
> translate AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED to the new flag in
> amdgpu_gem_create_ioctl, and not expose the new flag to userspace.
> 
> 
> But other than the question in my followup to the cover letter, this
> patch looks good to me as is.
> 
> 
> -- 
> Earthling Michel Dänzer               |               http://www.amd.com
> Libre software enthusiast             |             Mesa and X developer
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