Long radeon stalls on recent kernels

[Michel Dänzer]

On 11.12.2014 05:28, Andy Lutomirski wrote:
> On Wed, Dec 10, 2014 at 1:44 AM, Michel Dänzer <michel at daenzer.net> wrote:
>> On 10.12.2014 06:39, Andy Lutomirski wrote:
>>> On Tue, Dec 9, 2014 at 8:06 AM, Andy Lutomirski <luto at amacapital.net> wrote:
>>>> On Tue, Dec 9, 2014 at 1:18 AM, Michel Dänzer <michel at daenzer.net> wrote:
>>>>> On 09.12.2014 09:24, Andy Lutomirski wrote:
>>>>>>
>>>>>> The relevant line from latencytop seems to be:
>>>>>>
>>>>>> 154 20441402 489139 radeon_fence_default_wait [radeon]
>>>>>> fence_wait_timeout ttm_bo_wait [ttm] ttm_bo_move_accel_cleanup [ttm]
>>>>>> radeon_move_blit.isra.12 [radeon] radeon_bo_move [radeon]
>>>>>> ttm_bo_handle_move_mem [ttm] ttm_bo_evict [ttm] ttm_mem_evict_first
>>>>>> [ttm] ttm_bo_mem_space [ttm] ttm_bo_validate [ttm]
>>>>>> radeon_bo_fault_reserve_notify [radeon]
>>>>>
>>>>> Which process is this?
>>>>
>>>> Xorg
>>>>
>>>>>
>>>>> Looks like CPU access to a BO in VRAM, but the BO is located outside of
>>>>> the CPU visible area of VRAM, so it has to be moved into the CPU visible
>>>>> area first.

[...]

>>> But I'm still waiting for the day that buggy userspace *can't* cause
>>> kernel graphics stalls.
>>
>> Actually, this looks more like buggy userspace stalling itself. :)
> 
> I thought the stall was the kernel evicting things from vram.  Why
> does it need to wait for userspace for that?  Is it that userspace is
> actively using whatever's being evicted?

As I explained above, the stall happens because userspace does CPU
access to a BO which resides in the CPU-inaccessible part of VRAM. The
kernel has to move the BO into the CPU accessible part of VRAM before it
can let userspace proceed.

Current Mesa (10.4 or newer I think) sets a hint for BOs which will
likely be accessed by the CPU, so recent kernels can prioritize putting
those into the CPU accessible part of VRAM in the first place.

Or, if you're using EXA, the problem could be in the xf86-video-ati EXA
code.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer