** Changed in: linux (Ubuntu)
       Status: In Progress => Fix Released

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  More migrations with constant load

Status in The Ubuntu-power-systems project:
  Fix Released
Status in linux package in Ubuntu:
  Fix Released
Status in linux source package in Zesty:
  Fix Released

Bug description:
  == SRU Justification ==
  There is a significantly higher number of task migrations when the load is 
  fixed and not balanced across cores.

  Benchmark results are posted in the bug description and in the commits
  git log.

  This bug is resolved by mainline commit 
05b40e057734811ce452344fb3690d09965a7b6a, which is 
  in mailine as of 4.12-rc1.

  == Fix ==
  commit 05b40e057734811ce452344fb3690d09965a7b6a
  Author: Srikar Dronamraju <sri...@linux.vnet.ibm.com>
  Date:   Wed Mar 22 23:27:50 2017 +0530

      sched/fair: Prefer sibiling only if local group is under-utilized

  == Regression Potential ==
  Medium, since this commit does touch the scheduler.  However, the commit only 
makes a change to 
  allow a local group to pull a task, if the source group has more number of 
  tasks than the local group.

  == Test Case ==
  A test kernel was built with this patch and tested by the original bug 
  The bug reporter states the test kernel resolved the bug.

  == 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

  == 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")
  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

  == 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.

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