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I have a new installation of Debian 10 Buster for ARM (armhf) on a Cubietruck (aka Cubieboard 3, http://cubieboard.org/model/). The base system was installed on 2020-01-04 and is up to date. Note that the Cubietruck uses an Allwinner A20 SoC, and the bug seems to be related to this very hardware.
Debian 8 worked perfectly on the Cubietruck, but when I upgraded to Debian 10, a process called kworker/1:x-events
takes 8% CPU in the long term average and, about every 10 seconds, blocks hard disk IO so that the system becomes unusable.
I tried the following:
su root
apt-get install linux-perf
perf record -a -g sleep 10 # writes perf.data into the current directory
perf report
This shows the processes taking up most CPU time. Here is the top of the list
Samples: 2K of event 'cycles:ppp', Event count (approx.): 532078838
Children Self Command Shared Object Symbol
+ 89.45% 0.00% kworker/1:5-eve [kernel.kallsyms] [k] ret_from_fork
+ 89.45% 0.00% kworker/1:5-eve [kernel.kallsyms] [k] kthread
+ 89.45% 0.01% kworker/1:5-eve [kernel.kallsyms] [k] worker_thread
+ 89.34% 0.03% kworker/1:5-eve [kernel.kallsyms] [k] process_one_work
+ 89.23% 0.00% kworker/1:5-eve [drm_kms_helper] [k] output_poll_execute
+ 89.23% 0.00% kworker/1:5-eve [drm_kms_helper] [k] drm_helper_probe_detect_ctx
+ 86.17% 32.82% kworker/1:5-eve [sun4i_drm_hdmi] [k] sun4i_hdmi_connector_detect
+ 55.74% 44.12% kworker/1:5-eve [kernel.kallsyms] [k] ktime_get
+ 11.79% 6.71% kworker/1:5-eve [kernel.kallsyms] [k] arch_counter_read
+ 6.39% 6.39% kworker/1:5-eve [kernel.kallsyms] [k] arch_counter_get_cntpct
[...]
This was just a minute after a reboot, and the kworker-1:5-events
clearly takes too much CPU time. Apparently, there are some kernel functions that are out of control here. From the statistics, the candidates are sun4i_hdmi_connector_detect
or ktime_get
.
I looked for kernel modules with names similar to the offending functions. When I created a file /etc/modprobe.d/blacklist.conf
with a single line
blacklist sun4i_drm_hdmi
the problem disappeared. So I am pretty sure it is this module that malfunctions on the Cubietruck.
For me, the problem is solved here (because the Cubietruck does not have any HDMI and I could not care less about that kernel module), but I'd like to leave an online record of this fix.
Kind regards,
HPF