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

Adrian Hunter
2023-04-05T16:26:11Z

On 5/04/23 16:20, Arnaldo Carvalho de Melo wrote:
> Em Wed, Apr 05, 2023 at 11:47:26AM +0300, Adrian Hunter escreveu:
>> On 4/04/23 21:54, Arnaldo Carvalho de Melo wrote:
>>> 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)?
>
>> So I checked, and struct map *is* copy-on-write in
>> maps__fixup_overlappings(), so that should not be a problem.
>
>>>>> For struct maps, referencing it while simultaneously processing
>>>>> events seems to make even less sense?
>
>>>> Can you elaborate some more?
>
>> Only that the maps are not necessarily stable e.g. the map that you
>> need has been replaced in the meantime.
>
> Well, it may be sliced in several or shrunk by new ones overlapping it,
> but it if completely disappears, say a new map starts before the one
> disappearing and ends after it, then it remains with reference counts if
> there are hist_entries (or other data structure) pointing to them,
> right?
>
>> But upon investigation, the only user at the moment is
>> maps__find_ams(). If we kept the removed maps (we used to),
>> it might be possible to make maps__find_ams() work correctly
>> in any case.
>
> Humm, I think I see what you mean, maps__find_ams() is called when we
> are annotating a symbol, not when we're processing a sample, so it may
> be the case that at the time of annotation the executable that is being
> found (its parsing the target IP of a 'call' assembly instruction) was
> replaced, is that the case?

Yes, that is the possibility