Re: [PATCH] PowerPC: Program Priority Register support

[Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>]

On 23-05-2013 11:39, Ryan S. Arnold wrote:
> On Thu, May 23, 2013 at 9:05 AM, Adhemerval Zanella
> <azanella@linux.vnet.ibm.com> wrote:
>> Hi, some time ago I proposed a patch to save/restore the PowerPC Program
>> Priority Register along with platform specific functions to set its value.
>> However I later found out due a kernel limitation observed in the testcase
>> (the value was reset to default value in every interruption) I didn't try
>> to push.
>>
>> However now with Linux kernel 3.9 the PPR save/restore is implemented correctly
>> within the kernel. So I'm simplifying the patch to just add the platform inline
>> function to adjust the PPR value. The instruction are just nops on architectures
>> that does not support PPR (pre-ISA 2.05) and on older kernel it will adjust the
>> PPR on current process, but its value will be set to reset on a interrupt.
>>
>> Any comments, tips, advices?
>>
>> ---
>>
>> 2013-05-23  Adhemerval Zanella  <azanella@linux.vnet.ibm.com>
>>
>>         * manual/platform.texi: Add PowerPC PPR function set documentation.
>>         * sysdeps/powerpc/sys/platform/ppc.h: Add PowerPC PPR set function
>>         implementation.
>>
>> --
>>
>> diff --git a/manual/platform.texi b/manual/platform.texi
>> index e387ff4..9803ba8 100644
>> --- a/manual/platform.texi
>> +++ b/manual/platform.texi
>> @@ -34,3 +34,24 @@ This frequency is not related to the processor clock or the bus clock.
>>  It is also possible that this frequency is not constant.  More information is
>>  available in @cite{Power ISA 2.06b - Book II - Section 5.2}.
>>  @end deftypefun
>> +
>> +@deftypefun {void} __ppc_set_ppr_med (void)
>> +Set the Program Priority Register to medium value (default).
>> +
>> +The @dfn{Program Priority Register} (PPR) is a 64-bit register that controls
>> +the program's priority. By adjusting the PPR value the programmer may
>> +improve system throughput by causing the system resources to be used
>> +more efficiently, especially in contention situations.
>> +The three unprivileged states available are covered by the functions
>> +__ppc_set_ppr_med (medium - default), __ppc_set_ppc_low (low) and
>> +__ppc_set_ppc_med_low (medium low).  More information
>> +available in @cite{Power ISA 2.06b - Book II - Section 3.1}.
>> +@end deftypefun
>> +
>> +@deftypefun {void} __ppc_set_ppr_low (void)
>> +Set the Program Priority Register to low value.
>> +@end deftypefun
>> +
>> +@deftypefun {void} __ppc_set_ppr_med_low (void)
>> +Set the Program Priority Register to medium low value.
>> +@end deftypefun
>> diff --git a/sysdeps/powerpc/sys/platform/ppc.h b/sysdeps/powerpc/sys/platform/ppc.h
>> index 740831e..7cc0f1c 100644
>> --- a/sysdeps/powerpc/sys/platform/ppc.h
>> +++ b/sysdeps/powerpc/sys/platform/ppc.h
>> @@ -50,4 +50,34 @@ __ppc_get_timebase (void)
>>  #endif
>>  }
>>
>> +
>> +/* ISA 2.05 and beyond support the Program Priority Register (PPR) to adjust
>> +   thread priorities based on lock acquisition, wait and release. The ISA
>> +   defines the use of form 'or Rx,Rx,Rx' as the way to modify the PRI field.
>> +   The unprivileged priorities are:
>> +     Rx = 1 (low)
>> +     Rx = 2 (medium)
>> +     Rx = 6 (medium-low/normal)
>> +   The 'or' instruction form is a nop in previous hardware, so it is safe to
>> +   use unguarded. The default value is 'medium'.
>> + */
> You mention that Linux 3.9 supports PPR save/restore.  Did it also
> redefine 'normal' to Rx=2 as we'd hoped/asked?

>From comment at arch/powerpc/include/asm/ppc_asm.h the default PPR value is 3, which
correspond to Rx=6 (from ISA, check Book II, Chapter 3.2 'or' instruction).


>
> I suppose it doesn't make sense to have a test-case because an pre 3.9
> kernel might task switch after a set.
>
> Is there any sense/use in having a 'get' API?
>
> Ryan
>
Right now I don't see a compelling user case. The kernel, for instance, boosts the priority
to 4 (Rx=2) during exceptions without consulting the previous one. Usually use will use it
in specific cases to minimize contention: it will set priority to medium on a high-priority
task or low otherwise.