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

[Namhyung Kim]

Hi Adrian,

On Mon, Dec 29, 2014 at 02:58:12PM +0200, Adrian Hunter wrote:
> On 27/12/14 07:28, Namhyung Kim wrote:
> > 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,
> 
> Probably need .exclude_kernel = 1, here

Ah, right.

> 
> >>> +       };
> >>> +       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.
> 
> perf_evlist__set_tracking_event() also ensures there is only one tracking
> event so it is easy to identify. It is the only event with attr->mmap etc
> set to 1. Then you can use perf_evlist__add().

Well, yes, I think we can put the dummy tracking event anywhere in the
evlist with this, but I still slightly prefer put it at a fixed
location for a possible code simplification..

Thanks,
Namhyung
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