Subject: [v2 PATCH 0/2] powerpc: Detection and scheduler optimization for POWER9 bigcore

From: "Gautham R. Shenoy" <[email-masked]>
Date: Tue, 3 Jul 2018 16:33:49 +0530

From: "Gautham R. Shenoy" <[email-masked]>

Hi,

This is the second iteration of the patchset to add support for big-core on POWER9.

The earlier version can be found here: https://lkml.org/lkml/2018/5/11/245.

The changes from the previous version:
    - Added comments explaining the "ibm,thread-groups" device tree property.
    - Uses cleaner device-tree parsing functions to parse the u32 arrays.
    - Adds a sysfs file listing the small-core siblings for every CPU.
    - Enables the scheduler optimization by setting the CPU_FTR_ASYM_SMT bit
      in the cur_cpu_spec->cpu_features on detecting the presence
      of interleaved big-core.
    - Handles the corner case where there is only a single thread-group
      or when there is a single thread in a thread-group.

Description:
~~~~~~~~~~~~~~~~~~~~
A pair of IBM POWER9 SMT4 cores can be fused together to form a
big-core with 8 SMT threads. This can be discovered via the
"ibm,thread-groups" CPU property in the device tree which will
indicate which group of threads that share the L1 cache, translation
cache and instruction data flow.  If there are multiple such group of
threads, then the core is a big-core. Furthermore, the thread-ids of
such a big-core is obtained by interleaving the thread-ids of the
component SMT4 cores.

Eg: Threads in the pair of component SMT4 cores of an interleaved
big-core are numbered {0,2,4,6} and {1,3,5,7} respectively.
    
On such a big-core, when multiple tasks are scheduled to run on the
big-core, we get the best performance when the tasks are spread across
the pair of SMT4 cores.

The Linux scheduler supports a flag called "SD_ASYM_PACKING" which
when set in the SMT sched-domain, biases the load-balancing of the
tasks on the smaller numbered threads in the core. On an big-core
whose threads are interleavings of the threads of the small cores,
enabling SD_ASYM_PACKING in the SMT sched-domain automatically results
in spreading the tasks uniformly across the associated pair of SMT4
cores, thereby yielding better performance.

This patchset contains two patches which on detecting the presence of
interleaved big-cores will enable the the CPU_FTR_ASYM_SMT bit in the
cur_cpu_spec->cpu_feature.

Patch 1: adds support to detect the presence of
big-cores and reports the small-core siblings of each CPU X
via the sysfs file "/sys/devices/system/cpu/cpuX/big_core_siblings".

Patch 2: checks if the thread-ids of the component small-cores are
interleaved, in which case we enable the the CPU_FTR_ASYM_SMT bit in
the cur_cpu_spec->cpu_features which results in the SD_ASYM_PACKING
flag being set at the SMT level sched-domain.

Results:
~~~~~~~~~~~~~~~~~
Experimental results for ebizzy with 2 threads, bound to a single big-core
show a marked improvement with this patchset over the 4.18-rc3 vanilla
kernel.

The result of 100 such runs for 4.18-rc3 kernel and the 4.18-rc3 +
big-core-patches are as follows

4.18-rc3:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        records/s    :  # samples  : Histogram
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~	
[0 -       1000000]  :      0      : #
[1000000 - 2000000]  :      3      : #
[2000000 - 3000000]  :      16     : ####
[3000000 - 4000000]  :      11     : ###
[4000000 - 5000000]  :      0      : #
[5000000 - 6000000]  :      70     : ###############

4.18-rc3 + big-core-patches
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        records/s    :  # samples  : Histogram
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~	
[0 -       1000000]  :      0      : #
[1000000 - 2000000]  :      0      : #
[2000000 - 3000000]  :      6      : ##
[3000000 - 4000000]  :      0      : #
[4000000 - 5000000]  :      2      : #
[5000000 - 6000000]  :      92     : ###################

Gautham R. Shenoy (2):
  powerpc: Detect the presence of big-cores via "ibm,thread-groups"
  powerpc: Enable CPU_FTR_ASYM_SMT for interleaved big-cores

 Documentation/ABI/testing/sysfs-devices-system-cpu |   8 +
 arch/powerpc/include/asm/cputhreads.h              |  22 +++
 arch/powerpc/kernel/setup-common.c                 | 177 ++++++++++++++++++++-
 arch/powerpc/kernel/sysfs.c                        |  35 ++++
 4 files changed, 241 insertions(+), 1 deletion(-)

-- 
1.9.4