[PATCH v2] selftests: powerpc: Fix online CPU selection

[Michael Ellerman]

Sandipan Das <sandipan at linux.ibm.com> writes:
> The size of the CPU affinity mask must be large enough for
> systems with a very large number of CPUs. Otherwise, tests
> which try to determine the first online CPU by calling
> sched_getaffinity() will fail. This makes sure that the size
> of the allocated affinity mask is dependent on the number of
> CPUs as reported by get_nprocs().
>
> Fixes: 3752e453f6ba ("selftests/powerpc: Add tests of PMU EBBs")
> Reported-by: Shirisha Ganta <shiganta at in.ibm.com>
> Signed-off-by: Sandipan Das <sandipan at linux.ibm.com>
> Reviewed-by: Kamalesh Babulal <kamalesh at linux.vnet.ibm.com>
> ---
> Previous versions can be found at:
> v1: https://lore.kernel.org/linuxppc-dev/20200608144212.985144-1-sandipan@linux.ibm.com/
>
> Changes in v2:
> - Added NULL check for the affinity mask as suggested by Kamalesh.
> - Changed "cpu set" to "CPU affinity mask" in the commit message.

This sometimes breaks, eg:

  # ./count_instructions 
  test: count_instructions
  tags: git_version:v5.8-rc2-327-g9a1d992a7eb7
  sched_getaffinity: Invalid argument
  [FAIL] Test FAILED on line 123
  failure: count_instructions


This system has a messed up SMT setup, but the old code was able to cope
with it:

  # ppc64_cpu --info
  Core   0:    0*    1*    2     3     4     5     6     7  
  Core   1:    8     9    10*   11*   12    13    14    15  
  Core   2:   16    17    18    19    20    21    22    23  
  Core   3:   24    25    26    27    28    29    30    31  
  Core   4:   32    33    34    35    36    37    38    39  
  Core   5:   40    41    42    43    44    45    46    47  
  Core   6:   48    49    50    51    52    53    54    55  
  Core   7:   56    57    58    59    60    61    62    63  
  Core   8:   64    65    66    67    68    69    70    71  
  Core   9:   72    73    74    75    76    77    78    79  
  Core  10:   80    81    82    83    84    85    86    87  
  Core  11:   88    89    90    91    92    93    94    95  
  Core  12:   96    97    98    99   100*  101*  102*  103* 
  Core  13:  104*  105*  106*  107*  108*  109*  110*  111* 
  Core  14:  112*  113*  114*  115*  116*  117*  118*  119* 
  Core  15:  120   121   122   123   124   125   126   127  
  Core  16:  128   129   130   131   132   133   134   135  
  Core  17:  136   137   138   139   140   141   142   143  
  Core  18:  144   145   146   147   148   149   150   151  
  Core  19:  152   153   154   155   156   157   158   159 

cheers