[PATCH] powerpc: Avoid taking a data miss on every userspace instruction miss

[Christophe LEROY]

Hi Anton,

Le 04/04/2017 à 00:00, Anton Blanchard a écrit :
> Hi Christophe,
>
>>> -	if (user_mode(regs))
>>> +	if (!is_exec && user_mode(regs))
>>
>> Shouldn't it also check 'is_write' ?
>> If it is a store, is_write should be set, shouldn't it ?
>
> Thanks, Ben had the same suggestion. I'll add that further optimisation
> in a subsequent patch.
>
> Anton
>

For your information, I made some benchmark test using 'perf stat' with 
your app on MPC8321 and MPC885, and I got the following results:

MPC8321 before the change:

  Performance counter stats for './fault 1000' (10 runs):

        4491.971466      cpu-clock (msec) 
               ( +-  0.03% )
              47386      faults 
               ( +-  0.02% )

        4.727864465 seconds time elapsed 
          ( +-  0.17% )


MPC8321 after your change:

  Performance counter stats for './fault 1000' (10 runs):

        4278.738845      cpu-clock (msec) 
               ( +-  0.02% )
              35181      faults 
               ( +-  0.02% )

        4.504443891 seconds time elapsed 
          ( +-  0.19% )


MPC8321 after changing !is_exec by is_write

  Performance counter stats for './fault 1000' (10 runs):

        4268.187261      cpu-clock (msec) 
               ( +-  0.03% )
              35181      faults 
               ( +-  0.01% )

        4.489207922 seconds time elapsed 
          ( +-  0.20% )





MPC885 before the change:

  Performance counter stats for './fault 500' (10 runs):

          726605854      cpu-cycles 
               ( +-  0.03% )
             176067      dTLB-load-misses 
               ( +-  0.08% )
              52722      iTLB-load-misses 
               ( +-  0.01% )
              25718      faults 
               ( +-  0.03% )

        5.795924654 seconds time elapsed 
          ( +-  0.14% )


MPC885 after your change:

  Performance counter stats for './fault 500' (10 runs):

          711233251      cpu-cycles 
               ( +-  0.04% )
             152462      dTLB-load-misses 
               ( +-  0.09% )
              52715      iTLB-load-misses 
               ( +-  0.01% )
              19611      faults 
               ( +-  0.02% )

        5.673784606 seconds time elapsed 
          ( +-  0.14% )


MPC885 after changing !is_exec by is_write

  Performance counter stats for './fault 500' (10 runs):

          710904083      cpu-cycles 
               ( +-  0.05% )
             147162      dTLB-load-misses 
               ( +-  0.06% )
              52716      iTLB-load-misses 
               ( +-  0.01% )
              19610      faults 
               ( +-  0.02% )

        5.672091139 seconds time elapsed 
          ( +-  0.15% )



Christophe