Large spikes in latencies

[Yogi A. Patel]

> On 05/28/2015 08:54 PM, Michel Dänzer wrote:
>> On 28.05.2015 22:21, Yogi A. Patel wrote:
>> Setup - I am using a custom-compiled Linux kernel 3.14.17 on Ubuntu
>> 14.04. The kernel is patched with Xenomai using Ipipe
>> 3.14.17-x84-4.patch. I have a Radeon HD 8570HD R7 240 graphics card in
>> this system.
>> 
>> I can confirm that the radeon driver is compiled and loaded, and by
>> running “sudo lshw -C Display”, I can verify that my graphics card is
>> using the Radeon driver.
>> 
>> Problem - Now, when I run the latency test in Xenomai, I obtain large
>> latencies - up to 40us. If I blacklist the radeon, drm, drm_kms_help,
>> ttm, i2c_algo_bit kernel modules, and run the latency test with gdm not
>> running, the latencies are under 10us.
>> 
>> In previous kernel versions, such as 3.8.13, I have never seen such big
>> differences in latency results between gdm running/not running.
> Can you narrow down when the problem was introduced in the kernel,
> ideally with git bisect?
> 
> Assuming DPM is enabled, have you tried disabling that with radeon.dpm=0
> on the kernel command line?
> 
> Please attach the dmesg output pertaining to drm/radeon.
I did a few experiments and here is an update:

+ It seems that whatever is causing this is from drm/radeon and it came in sometime between 3.8.13 and 3.10.32. The dmesg output from 3.10.32 is attached, too

+ tests run with Radeon.dpm=0 still show the same issues.

+ the issue is not due to the model of the card. I swapped out my 8470 and tried a 70xx card and see the same latency issues.

+ the latencies are worse on 3.14 (40us) than on 3.10 (20-30us) regardless of the card.

+ latencies on 3.8 for both cards are below 10us. 

At this moment, I'm not sure I see exactly what is causing the issue or what else to try.