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[Rafael David Tinoco <rafael.tinoco@xxxxxxxxxxxxx>]

This systemtap script will get the execution path when it reaches set_task_cpu() in kernel 3.19.
set_task_cpu() is where perf sw cpu migration counter is updated.

#!/usr/bin/stap

probe kernel.function("set_task_cpu").call
{
    // running process
    // pr_name = execname()
    // pr_pid = pid()

    // task to be migrated
    _pid  = task_pid($p)
    _name = task_execname($p)
    _migr = $p->se->nr_migrations
    _ocpu = task_cpu($p)
    _ncpu = $new_cpu

    if (_ocpu != _ncpu) {
	printf("task %d (%s) (migr=%d) from (cpu=%d) to (cpu = %d)\n", _pid , _name , _migr, _ocpu, _ncpu)
	print_backtrace()
    }
}

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https://bugs.launchpad.net/bugs/1575407

Title:
  Trusty + 3.19 (lts-vivid) PERF wrong cpu-migration counter

Status in linux package in Ubuntu:
  Confirmed

Bug description:
  It was brought to my attention that:

  In a PowerPC based server, PERF seems to acuse cpu-migrations when
  only a single cpu is activated.

  ## perf

  Performance counter stats for 'CPU(s) 0':

  15027.888988      task-clock (msec)                 #    1.000 CPUs utilized [100.00%]
  25,206                     context-switches                 #    0.002 M/sec [100.00%]
  3,518                       cpu-migrations                     #    0.234 K/sec [100.00%]
  639                           page-faults                           #    0.043 K/sec                  
  41,545,780,384    cycles                                      #    2.765 GHz [66.68%]
  2,868,753,319       stalled-cycles-frontend    #    6.91% frontend cycles idle [50.01%]
  30,162,193,535    stalled-cycles-backend     #   72.60% backend  cycles idle [50.01%]
  11,161,722,533    instructions                          #    0.27  insns per cycle        
                                                                                     #    2.70  stalled cycles per insn [66.68%]
  1,544,072,679      branches                                #  102.747 M/sec [49.99%]
  52,536,867            branch-misses                     #    3.40% of all branches [49.99%]

  15.027768835 seconds time elapsed

  ## lscpu

  Architecture:          ppc64le
  Byte Order:            Little Endian
  CPU(s):                128
  On-line CPU(s) list:   0
  Off-line CPU(s) list:  1-127
  Thread(s) per core:    1
  Core(s) per socket:    1
  Socket(s):             1
  NUMA node(s):          2
  Model:                 8335-GCA
  L1d cache:             64K
  L1i cache:             32K
  L2 cache:              512K
  L3 cache:              8192K
  NUMA node0 CPU(s):     0
  NUMA node8 CPU(s):     

  So either task migrations are being done to offline cpus or perf is
  accounting it wrong.

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