Re: Wrong Perf Backtraces

[ahmadkhorrami <ahmadkhorrami@ut.ac.ir>]

Thanks.
On 2020-03-31 19:59, Milian Wolff wrote:

> 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.
> 
> Cheers