Re: [PATCH 02/37] perf record: Use a software dummy event to track task/mmap events

[Namhyung Kim]

Hi David,

On Sat, Dec 27, 2014 at 1:27 AM, David Ahern <dsahern@xxxxxxxxx> wrote:
> On 12/24/14 12:14 AM, Namhyung Kim wrote:
>>
>> Prepend a software dummy event into evlist to track task/comm/mmap
>> events separately.  This is a preparation of multi-file/thread support
>> which will come later.
>
>
> Are you are making this the first event because of how perf internals are
> coded -- that the first event tracks tasks events? With the tracking bit in
> evsel you should not need to do that. Is there another reason?

Yeah, I know the tracking bit can be set to any evsel in the evlist.
But I'd like to keep it at a fixed index so that it can be easily
identified at later stages (like perf report) too.  Ideally, it'd be
great if we have a way to distinguish this auto-added dummy tracking
event from other (user-added) (dummy?) tracking events if any.

>
> ---8<---
>
>> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
>> index cfbe2b99b9aa..72dff295237e 100644
>> --- a/tools/perf/util/evlist.c
>> +++ b/tools/perf/util/evlist.c
>> @@ -193,6 +193,44 @@ int perf_evlist__add_default(struct perf_evlist
>> *evlist)
>>         return -ENOMEM;
>>   }
>>
>> +int perf_evlist__prepend_dummy(struct perf_evlist *evlist)
>> +{
>> +       struct perf_event_attr attr = {
>> +               .type = PERF_TYPE_SOFTWARE,
>> +               .config = PERF_COUNT_SW_DUMMY,
>> +       };
>> +       struct perf_evsel *evsel, *pos;
>> +
>> +       event_attr_init(&attr);
>> +
>> +       evsel = perf_evsel__new(&attr);
>> +       if (evsel == NULL)
>> +               goto error;
>> +
>> +       /* use strdup() because free(evsel) assumes name is allocated */
>> +       evsel->name = strdup("dummy");
>> +       if (!evsel->name)
>> +               goto error_free;
>> +
>> +       list_for_each_entry(pos, &evlist->entries, node) {
>> +               pos->idx += 1;
>> +               pos->tracking = false;
>> +       }
>> +
>> +       list_add(&evsel->node, &evlist->entries);
>> +       evsel->idx = 0;
>> +       evsel->tracking = true;
>
>
> perf_evlist__set_tracking_event()?

I found that after I wrote this, so yes, it can use the function
instead of the oped-code.  But the loop traversal is needed anyway to
fixup the evsel->idx.

Thanks,
Namhyung
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