Re: [PATCH v5 14/29] x86,fs/resctrl: Support binary fixed point event counters

[Reinette Chatre]

Hi Tony,

On 6/6/25 9:25 AM, Luck, Tony wrote:
> On Tue, Jun 03, 2025 at 08:49:08PM -0700, Reinette Chatre wrote:
>>> +	sprintf(buf, "%0*llu", fp->decplaces, frac);
>>
>> I'm a bit confused here. I see fp->decplaces as the field width and the "0" indicates
>> that the value is zero padded on the _left_. I interpret this to mean that, for example,
>> if the value of frac is 42 then it will be printed as "0042". The fraction's value is modified
>> (it is printed as "0.0042") and there are no trailing zeroes to remove. What am I missing?
> 
> An example may help. Suppose architecture is providing 18 binary place
> numbers, and delivers the value 0x60000 to be displayed. With 18 binary
> places filesystem chooses 6 decimal places (I'll document the rationale
> for this choice in comments in next version). In binary the value looks
> like this:
> 
> 	integer	binary_places
> 	1	100000000000000000
> 
> Running through my algorithm will end with "frac" = 500000 (decimal).
> 
> Thus there are *trailing* zeroes. The value should be displayed as
> "1.5" not as "1.500000".

Instead of a counter example, could you please make it obvious through
the algorithm description and/or explanation of decimal place choice how
"frac" is guaranteed to never be smaller than "decplaces"?

Reinette