Re: [PATCH 05/37] perf tools: Create separate mmap for dummy tracking event

[Namhyung Kim]

On Fri, Dec 26, 2014 at 7:09 AM, Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> On Wed, Dec 24, 2014 at 04:15:01PM +0900, Namhyung Kim wrote:
>
> SNIP
>
>>
>>  union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx);
>> -
>>  void perf_evlist__mmap_consume(struct perf_evlist *evlist, int idx);
>> +struct perf_mmap *perf_evlist__mmap_desc(struct perf_evlist *evlist, int idx);
>>
>>  int perf_evlist__open(struct perf_evlist *evlist);
>>  void perf_evlist__close(struct perf_evlist *evlist);
>> @@ -211,6 +214,12 @@ bool perf_evlist__can_select_event(struct perf_evlist *evlist, const char *str);
>>  void perf_evlist__to_front(struct perf_evlist *evlist,
>>                          struct perf_evsel *move_evsel);
>>
>> +/* convert from/to negative idx for track mmaps */
>> +static inline int track_mmap_idx(int idx)
>> +{
>> +     return -idx - 1;
>> +}
>
> hum, whats the logic with negative numbers in here?
> you still access track_mmap array with this index no?

For each index (per-cpu or per-task depends on user input), it now has
two mmaps - one for normal (sampling) mmap and another for track
(metadata) mmap.  So I wanted to distinguish them by using positive
and negative number.  But simply changing the sign did not work due to
0 index.  And the subtracting by 1 works nicely for both direction.

0 => -1 => 0
1 => -2 => 1
2 => -3 => 2
3 => -4 => 3

So when it sees a negative index, it'll convert it using
track_mmap_idx() to get a matching (track) mmap.

Thanks,
Namhyung
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