0day has reported many strange performance changes (regression or...
improvement), in which there was no obvious relation between the culprit
commit and the benchmark at the first look, and it causes people to doubt
the test itself is wrong.
Upon further check, many of these cases are caused by the change to the
alignment of kernel text or data, as whole text/data of kernel are linked
together, change in one domain can affect alignments of other domains.
To help quickly identifying if the strange performance change is caused
by _data_ alignment, add a debug option to force the data sections from
all .o files aligned on THREAD_SIZE, so that change in one domain won't
affect other modules' data alignment.
We have used this option to check some strange kernel changes [1][2][3],
and those performance changes were gone after enabling it, which proved
they are data alignment related. Besides these publicly reported cases,
recently there are other similar cases found by 0day, and this option
has been actively used by 0Day for analyzing strange performance changes.
+ .data : AT(ADDR(.data) - LOAD_OFFSET)
+#ifdef CONFIG_DEBUG_FORCE_DATA_SECTION_ALIGNED
+ /* Use the biggest alignment of below sections */
+ SUBALIGN(THREAD_SIZE)
+#endif