Re: [PATCH v5 00/17] Reference count checker and related fixes

Arnaldo Carvalho de Melo
2023-04-04T18:54:38Z

Em Tue, Apr 04, 2023 at 03:41:38PM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Tue, Apr 04, 2023 at 08:25:41PM +0300, Adrian Hunter escreveu:
> > On 4/04/23 18:58, Ian Rogers wrote:
> > > Ping. It would be nice to have this landed or at least the first 10
> > > patches that refactor the map API and are the bulk of the
> > > lines-of-code changed. Having those landed would make it easier to
> > > rebase in the future, but I also think the whole series is ready to
> > > go.
> >
> > I was wondering if the handling of dynamic data like struct map makes
> > any sense at present. Perhaps someone can reassure me.
> >
> > A struct map can be updated when an MMAP event is processed. So it
>
> Yes, it can, and the update is made via a new PERF_RECORD_MMAP, right?
>
> So:
>
> perf_event__process_mmap()
> machine__process_mmap2_event()
> map__new() + thread__insert_map(thread, map)
> maps__fixup_overlappings()
> maps__insert(thread->maps, map);
>
> Ok, from this point on new samples on ] map->start .. map->end ] will
> grab a refcount to this new map in its hist_entry, right?
>
> When we want to sort by dso we will look at hist_entry->map->dso, etc.

And in 'perf top' we go decaying hist entries, when we delete the
hist_entry, drop the reference count to things it holds, that will then
be finally deleted when no more hist_entries point to it.

> > seems like anything racing with event processing is already broken, and
> > reference counting / locking cannot help - unless there is also
> > copy-on-write (which there isn't at present)?
>
> > For struct maps, referencing it while simultaneously processing
> > events seems to make even less sense?
>
> Can you elaborate some more?

- Arnaldo