Re: Wrong Perf Backtraces

Em Tue, Mar 31, 2020 at 05:29:17PM +0200, Milian Wolff escreveu:
> On Dienstag, 31. März 2020 17:02:37 CEST ahmadkhorrami wrote:
> > Hi Milian,
> > Thanks for the detailed answer. Well, the bug you mentioned is bad news.
> > Because I sample using uppp. Perhaps this leads to these weird traces.
> 
> Please read the full thread from here on:
> 
> https://lkml.org/lkml/2018/11/2/86
> 
> But as I said - it should be easy to check if this is really the issue are 
> running into or not: Try to see if you see the problem when you sample without 
> `ppp`. If not, then you can be pretty sure it's this issue. If you still see 
> it, then it's something different.
> 
> > Is this a purely software bug?
> 
> I wouldn't call it that, personally. Rather, it's a limitation in the hardware 
> and software. We would need something completely different to "fix" this, i.e. 
> something like a deeper LBR. That's btw another alternative you could try: 
> `perf record --call-graph lbr` and live with the short call stacks. But at 
> least these should be correct (afaik). For me personally they are always far 
> too short and thus not practical to use in reality.

Probably this may help:

  From: Kan Liang <[email protected]>
  Subject: [PATCH V4 00/17] Stitch LBR call stack (Perf Tools)
  Date: Thu, 19 Mar 2020 13:25:00 -0700
  https://lore.kernel.org/lkml/[email protected]/

- Arnaldo