Comment 0 for bug 1713576

== Comment: #0 - PUVICHAKRAVARTHY RAMACHANDRAN - 2017-08-06 13:44:45 ==
---Problem Description---
Significantly higher number of task migrations when the load is fixed but not balanced across cores.

---uname output---
Linux isvbos3 4.10.0-29-generic #33~16.04.1-Ubuntu SMP Tue Jul 25 18:17:06 UTC 2017 ppc64le ppc64le ppc64le GNU/Linux

---Additional Hardware Info---
Power9 dd2.0

Machine Type = Power9

---Steps to Reproduce---
 Benchmark : Multithreaded - cpu intensive. The system had 2 socket/ 32 cores/ SMT4 mode.

When 64 threads was run - the migrations were less over 10s interval.
when 80 threads were run - the migrations were very high.

Ideally, it should have been very minimal, as the over all load was constant

== Comment: #3 - SRIKAR DRONAMRAJU - 2017-08-11 06:56:47 ==
As suspected (commit : 05b40e0577 : "sched/fair: Prefer sibiling only if local group is under-utilized")
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=05b40e0577
should fix the problem

Ran ' perf stat -a -r 5 -e sched:sched_migrate_task /home/srikar/work/ebizzy-0.3/ebizzy -t 35 -S 100'
to detect the problem and verify the fix

Here is perf stat without fix.

Performance counter stats for 'system wide' (5 runs):

             7,758 sched:sched_migrate_task ( +- 1.28% )

     100.015658079 seconds time elapsed ( +- 0.00% )

perf stat with fix.

Performance counter stats for 'system wide' (5 runs):

               415 sched:sched_migrate_task ( +- 11.74% )

     100.016021787 seconds time elapsed ( +- 0.00% )

git describe on upstream kernel says v4.11-rc2
# git describe 05b40e0577
v4.11-rc2-227-g05b40e0

== Comment: #4 - SRIKAR DRONAMRAJU - 2017-08-11 07:05:37 ==
Attaching the patch that needs to be applied to fix this bug.
Verified that patch fixes the problem.