dd copy, context switch overhead

[Colin Ian King]

Hi,

I've observed that CONFIG_CONTEXT_TRACKING_FORCE is being turned on when
we have CONFIG_NO_HZ_FULL enabled.  The CONTEXT_TRACKING_FORCE is
causing some noticeable overhead, for example:

dd if=/dev/zero bs=$DD_BS count=$DD_COUNT | cat | cat | cat | dd
of=/dev/null

..this takes about 145 seconds with CONTEXT_TRACKING_FORCE enabled and
about 83 seconds with it disabled.

I suggest that we should revert back to CONFIG_NO_HZ_IDLE and make sure
CONFIG_CONTEXT_TRACKING_FORCE is disabled.

I believe this explains the huge increase in duration of the dd_copy
test in the QA tests, see the dd_copy graph and the blue line that shows
the duration (in seconds) of the copy:
http://reports.qa.ubuntu.com/power/hardware/arch/amd64/?size=big

Colin