linux-kernel - Re: [RESEND PATCH V5 1/2] perf/core: Add new branch sample type for HW index of raw branch records

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Date:   Mon, 20 Jan 2020 11:50:59 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     eranian@...gle.com, acme@...hat.com, mingo@...nel.org,
        mpe@...erman.id.au, linux-kernel@...r.kernel.org, jolsa@...nel.org,
        namhyung@...nel.org, vitaly.slobodskoy@...el.com,
        pavel.gerasimov@...el.com, ak@...ux.intel.com
Subject: Re: [RESEND PATCH V5 1/2] perf/core: Add new branch sample type for
 HW index of raw branch records



On 1/20/2020 4:23 AM, Peter Zijlstra wrote:
> On Thu, Jan 16, 2020 at 07:57:56AM -0800, kan.liang@...ux.intel.com wrote:
> 
>>   struct perf_branch_stack {
>>   	__u64				nr;
>> +	__u64				hw_idx;
>>   	struct perf_branch_entry	entries[0];
>>   };
> 
> The above and below order doesn't match.
> 
>> @@ -849,7 +853,11 @@ enum perf_event_type {
>>   	 *	  char                  data[size];}&& PERF_SAMPLE_RAW
>>   	 *
>>   	 *	{ u64                   nr;
>> -	 *        { u64 from, to, flags } lbr[nr];} && PERF_SAMPLE_BRANCH_STACK
>> +	 *        { u64 from, to, flags } lbr[nr];
>> +	 *
>> +	 *        # only available if PERF_SAMPLE_BRANCH_HW_INDEX is set
>> +	 *        u64			hw_idx;
>> +	 *      } && PERF_SAMPLE_BRANCH_STACK
> 
> That wants to be written as:
> 
> 		{ u64			nr;
> 		  { u64 from, to, flags; } entries[nr];
> 		  { u64	hw_idx; } && PERF_SAMPLE_BRANCH_HW_INDEX
> 		} && PERF_SAMPLE_BRANCH_STACK
> 
> But the big question is; why isn't it:
> 
> 		{ u64			nr;
> 		  { u64	hw_idx; } && PERF_SAMPLE_BRANCH_HW_INDEX
> 		  { u64 from, to, flags; } entries[nr];
> 		} && PERF_SAMPLE_BRANCH_STACK
> 
> to match the struct perf_branch_stack order. Having that variable sized
> entry in the middle just seems weird.


Usually, new data should be output to the end of a sample.
The comments and codes are all based on that way.
However, the entries[0] is sized entry, so I have to put the hw_idx 
before entry. It makes the inconsistency. Sorry for the confusion caused.

I will fix it in V6.

Thanks,
Kan

> 
>>   	 *
>>   	 * 	{ u64			abi; # enum perf_sample_regs_abi
>>   	 * 	  u64			regs[weight(mask)]; } && PERF_SAMPLE_REGS_USER