On 2018-12-06 15:31:50 [+0100], Luca Abeni wrote:

Update, after more tests:
- With all the "*DEBUG*" entries disabled in .config, "nohz=off" in
the kernel command line and the i915 driver disabled in .config, I
consistently get a worst-case latency below 40us (this is the order of
magnitude I was expecting). Good
- With everything as in the above configuration, but enabling the i915
driver in .config, I get a worst case latency of about 80us. Applying
Sebastian's patch does not seem to affect the latency. I also disabled
the various tracing options in .config, but the maximum latency is
still around 80us.

The patch should avoid the backtrace you sent at the begin of this
thread. This one happens otherwise if you enable the i915 events.

The latencies mentioned above have been measured with
  cyclictest -n -p 80 -i 50 -t 1 -m -a
while running "du -h /" and a "make -j 2" of the kernel in background.

So this should fire one thread on CPU0 up and I believe and pin it
there.
That interval looks very low. I would suggest to increment it around 200
or so.

So, the numbers I get now are much better, but the i915 driver still
seems to create some real-time issues.

The i915 could increase the latency a little. You could try to add
	i915.enable_dc=0 i915.enable_rc6=0

to disable power management on the GPU.

Luca

Sebastian