linux-kernel - Re: [PATCH v1 2/2] perf header: Support hybrid CPU_PMU

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Date:   Thu, 6 May 2021 22:43:39 +0800
From:   "Jin, Yao" <yao.jin@...ux.intel.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     acme@...nel.org, jolsa@...nel.org, peterz@...radead.org,
        mingo@...hat.com, alexander.shishkin@...ux.intel.com,
        Linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        kan.liang@...el.com, yao.jin@...el.com
Subject: Re: [PATCH v1 2/2] perf header: Support hybrid CPU_PMU_CAPS

Hi Jiri,

On 5/6/2021 9:22 PM, Jiri Olsa wrote:
> On Thu, May 06, 2021 at 12:59:08PM +0800, Jin, Yao wrote:
>> Hi Jiri,
>>
>> On 5/4/2021 11:07 PM, Jiri Olsa wrote:
>>> On Fri, Apr 30, 2021 at 03:46:02PM +0800, Jin Yao wrote:
>>>> On hybrid platform, it may have several cpu pmus, such as,
>>>> "cpu_core" and "cpu_atom". The CPU_PMU_CAPS feature in perf
>>>> header needs to be improved to support multiple cpu pmus.
>>>>
>>>> The new layout in header is defined as:
>>>>
>>>> <nr_caps>
>>>> <caps string>
>>>> <caps string>
>>>> <pmu name>
>>>> <nr of rest pmus>
>>>
>>> not sure why is the 'nr of rest pmus' needed
>>>
>>
>> The 'nr of rest pmus' indicates the remaining pmus which are waiting for process.
>>
>> For example,
>>
>> <nr_caps>
>> <caps string>
>> "cpu_core"
>> 1
>> <nr_caps>
>> <caps string>
>> "cpu_atom"
>> 0
>>
>> When we see '0' in data file processing, we know all the pmu have been processed yet.
>>
>>> the current format is:
>>>
>>>           u32 nr_cpu_pmu_caps;
>>>           {
>>>                   char    name[];
>>>                   char    value[];
>>>           } [nr_cpu_pmu_caps]
>>>
>>>
>>> I guess we could extend it to:
>>>
>>>           u32 nr_cpu_pmu_caps;
>>>           {
>>>                   char    name[];
>>>                   char    value[];
>>>           } [nr_cpu_pmu_caps]
>>> 	char pmu_name[]
>>>
>>>           u32 nr_cpu_pmu_caps;
>>>           {
>>>                   char    name[];
>>>                   char    value[];
>>>           } [nr_cpu_pmu_caps]
>>> 	char pmu_name[]
>>>
>>> 	...
>>>
>>> and we could detect the old format by checking that there's no
>>> pmu name.. but maybe I'm missing something, I did not check deeply,
>>> please let me know
>>>
>>
>> Actually we do the same thing, but I just add an extra 'nr of rest pmus'
>> after the pmu_name. The purpose of 'nr of rest pmus' is when we see '0' at
>> 'nr of rest pmus', we know that all pmus have been processed.
>>
>> Otherwise, we have to continue reading data file till we find something
>> incorrect and then finally drop the last read data.
> 
> you have the size of the feature data right? I think we use
> it in other cases to check if there are more data
> 

The challenge for us is if we need to compatible with the old perf.data which was generated by old 
perf tool.

For the old perf.data, the layout in header is:

nr of caps
caps string 1
caps string 2
...
caps string N

It doesn't carry with any other fields such as size of caps data.

To be compatible with old perf.data, so I have to extend the layout to:

nr of caps for pmu 1
caps string 1
caps string 2
...
caps string N
name of pmu 1
nr of rest pmus

nr of caps for pmu2
caps string 1
caps string 2
...
caps string N
name of pmu 2
nr of rest pmus

When the new perf tool detects the string such as "cpu_", it can know that it's the pmu name field 
in new perf.data, otherwise it's the old perf.data.

If we add new field such as "size" to the layout, I'm afraid the new perf tool can not process the 
old perf.data correctly.

If we don't need to support old perf.data, that makes things easy.

>>
>> So is this solution acceptable?
>>
>>> also would be great to move the format change and storing hybrid
>>> pmus in separate patches
>>>
>>
>> Maybe we have to put the storing and processing into one patch.
>>
>> Say patch 1 contains the format change and storing hybrid pmus. And patch 2
>> contains the processing for the new format. If the repo only contains the
>> patch 1, I'm afraid that may introduce the compatible issue.
> 
> maybe you can have change of caps format in one patch
> and storing/processing hybrid pmus in another?
> 

But there is no data structure defined in header.h for each feature.

It directly uses do_write/do_write_string in 'write()' ops to write the feature data.

So for the new layout, as I mentioned above, if we change the layout to

nr of caps for pmu 1
caps string 1
caps string 2
...
caps string N
"cpu"
0

We need to call do_write/do_write_string, actually it's the storing method. So I don't understand 
well for having changes of caps format in one patch, I'm sorry about that. :(

Thanks
Jin Yao

> jirka
>