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. > 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
