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

["Liang, Kan" <kan.liang@linux.intel.com>]

On 1/20/2020 3:24 PM, Peter Zijlstra wrote:
> On Mon, Jan 20, 2020 at 11:50:59AM -0500, Liang, Kan wrote:
>>
>>
>> On 1/20/2020 4:23 AM, Peter Zijlstra wrote:
>>> On Thu, Jan 16, 2020 at 07:57:56AM -0800, kan.liang@linux.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.
> 
> Because.... you want old tools to read new output?
>

Yes, for some cases, it helps.
If no other sample types are output after PERF_SAMPLE_BRANCH_STACK,
old perf tool will ignore the hw_idx.
But, if we also have to output other sample types, e.g 
PERF_SAMPLE_DATA_SRC or PERF_SAMPLE_PHYS_ADDR, which are output after 
PERF_SAMPLE_BRANCH_STACK. The hw_idx will mess them up.
Old perf tool doesn't work anymore.


>> However, the entries[0] is sized entry, so I have to put the hw_idx before
> 
> entries[0] is only in the C thing, and in C you indeed have to put
> hw_idx before.
> 
>> entry. It makes the inconsistency. Sorry for the confusion caused.
> 
> n/p it's clear now I think.

Should I send V6 patch to move hw_idx before entry as below?

@@ -853,7 +857,9 @@ enum perf_event_type {
          *        char                  data[size];}&& PERF_SAMPLE_RAW
          *
          *      { u64                   nr;
-        *        { u64 from, to, flags } lbr[nr];} && 
PERF_SAMPLE_BRANCH_STACK
+        *        { u64 hw_idx; } && PERF_SAMPLE_BRANCH_HW_INDEX
+        *        { u64 from, to, flags } lbr[nr];
+        *      } && PERF_SAMPLE_BRANCH_STACK
          *
          *      { u64                   abi; # enum perf_sample_regs_abi
          *        u64                   regs[weight(mask)]; } && 
PERF_SAMPLE_REGS_USER

@@ -6634,6 +6639,8 @@ void perf_output_sample(struct perf_output_handle 
*handle,
                              * sizeof(struct perf_branch_entry);

                         perf_output_put(handle, data->br_stack->nr);
+                       if (perf_sample_save_hw_index(event))
+                               perf_output_put(handle, 
data->br_stack->hw_idx);
                         perf_output_copy(handle, 
data->br_stack->entries, size);
                 } else {
                         /*



Thanks,
Kan