Re: [Xen-devel] [PATCH v4 10/11] x86/intel_pstate: support the use of intel_pstate in pmstat.c

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.09.15 at 07:35, <wei.w.wang@xxxxxxxxx> wrote:
> On 09/09/2015 23:55,  Jan Beulich wrote:
>>>> On 09.09.15 at 17:16, <wei.w.wang@xxxxxxxxx> wrote:
>> On 09/09/2015 21:12,  Jan Beulich wrote:
>>>>> On 09.09.15 at 14:56, <wei.w.wang@xxxxxxxxx> wrote:
>>> Can you please explain more why it doesn't scale? 
>>> From my point of view, any other future value representation can be 
>>> passed from the producer to the related consumer through this method.
>> 
>>> Did you read all of my earlier replies? I already said there "Just 
>>> consider
>> what happens to the code when we end up gaining a few
>>> more drivers providing percentage values, and perhaps another one 
>>> providing
>> a third variant of output representation."
>> 
>> Yes, I have read that. I am not sure if I got your point, but my 
>> meaning was when we add new drivers to the code, e.g. xx_pstate 
>> driver, we can still have the name, "xx_pstate", assigned to 
>> "p_cpufreq->scaling_driver" to distinguish one another. If the driver 
>> uses a different variant of output representation, which cannot be 
>> held by " uint32_t scaling_max_perf" (it needs "uint64_t" for example, then 
> that driver developer needs to add a new field here like  "
>> uint64_t scaling_max_perf_xx").
>> What is the scaling problem? 
> 
>>      if (strcmp() == 0 ||
>>          strcmp() == 0 ||
>>          strcmp() == 0) {
>>      ...
>>      } else if (strcmp() == 0) {
>>      ...
>>      } else {
>>      ...
>>      }
> 
>> is just ugly, and gets all the uglier the more strcmp()s get added.
>> Have a boolean or enumeration indicating what kind of data there is, and the 
> above changes to
> 
>>      switch (kind) {
>>      case absolute: ...
>>      case percentage: ...
>>      }
> 
> Ok. I will replace the default "scaling_driver[CPUFREQ_NAME_LEN]" with an 
> enum type, like this following
> ...
> - char scaling_driver[CPUFREQ_NAME_LEN];
> + enum scaling_driver_flag scaling_driver;
> ...
> 
> We cannot keep both of the above two there, because there is a 128Byte size 
> limit. Then somewhere, we need to translate the character-represented 
> scaling_driver to our new enum-represented scaling_driver. For example, in 
> pmstat.c, the following:
> 
> if ( cpufreq_driver->name[0] )
>         strlcpy(op->u.get_para.scaling_driver,
>             cpufreq_driver->name, CPUFREQ_NAME_LEN);
> else
>         strlcpy(op->u.get_para.scaling_driver, "Unknown", CPUFREQ_NAME_LEN);
> 
> needs to be changed to:
> if ( strncmp(cpufreq_driver->name[0], "intel_pstate", CPUFREQ_NAME_LEN) == 0 )
>     op->u.get_para.scaling_driver = INTEL_PSTATE;
> else if ( strncmp(cpufreq_driver->name[0], "acpi_cpufreq", CPUFREQ_NAME_LEN) 
> == 0 )
>     op->u.get_para.scaling_driver = ACPI_CPUFREQ;
> ...
> 
> Seems we still cannot get rid of these strncmp()s. Is this acceptable, or 
> should we change "struct cpufreq_driver" to use enum represented driver name 
> as well, or do you have a better suggestion?

The one I explained before: Express the data representation type
in an enum, not the driver kind. But even if we went with the above,
the strcmp() ugliness would at least be limited to the producer, not
enforced onto any number of consumers.

Jan


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