Large spikes in latencies

[Yogi A. Patel]

Hi -

Note - I know that this is not a Xenomai mailing list - however I have already posted there and it appears that the problems I am seeing are not from Xenomai, but rather from the use of the radeon driver.

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.

I originally thought that the radeon driver in the 3.14.17 kernel may not be the latest because the date on the radeon source in the kernel is 20080523 - but it seems like that date was just never updated (according to the answer to my previous question here <https://www.mail-archive.com/xorg-driver-ati@lists.x.org/msg26099.html>). So I assume the radeon source in the kernel is up-to-date (especially since it shows the driver supporting my graphics card).

The only odd things I see in dmesg is it says “drm taints the kernel; not creating trace events” The same message is present in dmesg for radeon.

Does anyone have any thoughts on what might be causing this or how to best debug it? I am quite lost since I don’t seem to see anything that sticks out.

Yogi
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