Re: [announce] Performance Counters for Linux, v6

[Ingo Molnar <mingo@elte.hu>]

* Corey Ashford <cjashfor@linux.vnet.ibm.com> wrote:

> Ingo Molnar wrote:
>> * stephane eranian <eranian@googlemail.com> wrote:
>>
>>> Hi,
>>>
>>> \x10Corey brings up an interesting problem which I wanted to comment on.
>>>
>>> The current proposal hinges on the idea that by interpreting a single 
>>> value the kernel can understand what the user wants to measure. For  
>>> instance, if I pass type=0, then the kernel understands I want to  
>>> measure CPU_CYCLES. Given that the number of events and their unit 
>>> mask combinations can be large, the proposal also provides a "raw" 
>>> mode, where the content of the type field is interpreted as the raw 
>>> value to put into a register.
>>>
>>> This is where there is an issue because with several PMU models,  
>>> including on X86, using the raw bit + 64 value is not enough to 
>>> figure out what the user wants to measure. This happens when the PMU 
>>> has more than counters. Thus, interpreting each raw value has the 
>>> event code may be wrong. To remain on familiar territory, the Nehalem 
>>> uncore PMU has an opcode matcher register, that uses a 64-bit value. 
>>> On AMD64 Family 10h, you have IBS. But I could give examples on 
>>> Itanium with opcode matchers, range restrictions. Corey provided 
>>> other examples for Power. The API has to provide a way to express 
>>> what the raw value is meant for: counter, matcher, filter...
>>
>> this can be done in a number of ways (in order of increasing levels of  
>> abstraction):
>>
>> - the raw type is kept wide enough. Paul already requested the raw type
>>   to be widened to 128 bits to express certain PowerPC features.
>>
>> - or the PMU capability is expressed as a special counter type (if it's
>>   useful enough) - and then either the write() method or ioctl is extended
>>   to express attributes we want to set/change while a counter is running.
>>
>> - or the highest level counter / hw event data type is extended with new
>>   attribute field(s).
>>
>> My feeling is that we generally want such hw features to start small -  
>> i.e. at the raw type level initially. Then we can allow them to climb 
>> the ladder, if they prove their utility in practice. We've got space 
>> reserved in the ABI to allow for growth like this.
>>
>> 	Ingo
>
>
> Hi Ingo and Stephane,
>
> Thanks for the replies.
>
> I think any one of those solutions would work for Power's Instruction 
> Matching Register.  If more than one register needs to be programmed, or 
> the values don't fit into the 128-bit raw event types, we could use the 
> "special counter" approach, I think.
>
> I will have another look at the Power PMU description and see if there 
> are other constraints that might cause us to want to go one way or the 
> other, or perhaps a different way.

thanks, that's really appreciated!

One useful approach would be to come up with a bitcount that you think 
would fit considering even (currently) fringe/odd features - and we'd make 
sure there's enough space for that in the ABI - should there be a 
need/desire to expose that in the future.

	Ingo