Re: [PATCH v2 2/2] powerpc/hv-24x7: Add sysfs files inside hv-24x7 device to show cpumask

[kajoljain <kjain@linux.ibm.com>]

On 6/26/20 1:15 PM, Gautham R Shenoy wrote:
> On Wed, Jun 24, 2020 at 05:58:31PM +0530, Madhavan Srinivasan wrote:
>>
>>
>> On 6/24/20 4:26 PM, Gautham R Shenoy wrote:
>>> Hi Kajol,
>>>
>>> On Wed, Jun 24, 2020 at 03:47:54PM +0530, Kajol Jain wrote:
>>>> Patch here adds a cpumask attr to hv_24x7 pmu along with ABI documentation.
>>>>
>>>> command:# cat /sys/devices/hv_24x7/cpumask
>>>> 0
>>> Since this sysfs interface is read-only, and the user cannot change
>>> the CPU which will be making the HCALLs to obtain the 24x7 counts,
>>> does the user even need to know if currently CPU X is the one which is
>>> going to make HCALLs to retrive the 24x7 counts ? Does it help in any
>>> kind of trouble-shooting ?
>> Primary use to expose the cpumask is for the perf tool.
>> Which has the capability to parse the driver sysfs folder
>> and understand the cpumask file. Having cpumask
>> file will reduce the number of perf commandline
>> parameters (will avoid "-C" option in the perf tool
>> command line). I can also notify the user which is
>> the current cpu used to retrieve the counter data.
> 
> Fair enough. Can we include this in the patch description ?

Sure will update in next version of patchset.

Thanks,
Kajol Jain

> 
>>
>>> It would have made sense if the interface was read-write, since a user
>>> can set this to a CPU which is not running user applications. This
>>> would help in minimising jitter on those active CPUs running the user
>>> applications.
>>
>> With cpumask backed by hotplug
>> notifiers, enabling user write access to it will
>> complicate the code with more additional check.
>> CPU will come to play only if the user request for
>> counter data. If not, then there will be no HCALLs made
>> using the CPU.
> 
> Well, I was wondering if you could make the interface writable because
> I couldn't think of the use of a read-only interface. With the
> perf-use case you have provided, I guess it makes sense. I am ok with
> it being a read-only interface.
> 
>>
>> Maddy
> 
> --
> Thanks and Regards
> gautham.
>