selftests/resctrl: Read in less obvious order to defeat prefetch optimizations
authorIlpo Järvinen <ilpo.jarvinen@linux.intel.com>
Fri, 15 Dec 2023 15:05:07 +0000 (17:05 +0200)
committerShuah Khan <skhan@linuxfoundation.org>
Tue, 13 Feb 2024 20:56:44 +0000 (13:56 -0700)
commitbcdb2e9d9f319935938a5addee040e37b60192ff
treecc96eca2a71d4e2af92c6ebacbbef3f28a09d742
parent90a009db09e2b91990df28e47844d44ae0ef9aa8
selftests/resctrl: Read in less obvious order to defeat prefetch optimizations

When reading memory in order, HW prefetching optimizations will
interfere with measuring how caches and memory are being accessed. This
adds noise into the results.

Change the fill_buf reading loop to not use an obvious in-order access
using multiply by a prime and modulo.

Using a prime multiplier with modulo ensures the entire buffer is
eventually read. 23 is small enough that the reads are spread out but
wrapping does not occur very frequently (wrapping too often can trigger
L2 hits more frequently which causes noise to the test because getting
the data from LLC is not required).

It was discovered that not all primes work equally well and some can
cause wildly unstable results (e.g., in an earlier version of this
patch, the reads were done in reversed order and 59 was used as the
prime resulting in unacceptably high and unstable results in MBA and
MBM test on some architectures).

Link: https://lore.kernel.org/linux-kselftest/TYAPR01MB6330025B5E6537F94DA49ACB8B499@TYAPR01MB6330.jpnprd01.prod.outlook.com/
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
tools/testing/selftests/resctrl/fill_buf.c