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 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
