Re: [PATCH dwarves] pahole: Avoid generating artificial inlined functions for BTF

[Alan Maguire <alan.maguire@oracle.com>]

On 21/10/2025 13:32, Jakub Sitnicki wrote:
> On Fri, Oct 03, 2025 at 10:36 AM -07, Yonghong Song wrote:
>> But actually, one of function 'foo' is marked as DW_INL_inlined which means
>> we should not treat it as an elf funciton. The patch fixed this issue
>> by filtering subprograms if the corresponding function__inlined() is true.
> 
> I have a semi-related question: are there any plans for BTF to indicate
> when a function has been inlined? Not necessarily where it has been
> inlined, just that it has, somewhere, at least once.
> 
> When tracing with bpftrace or perf without a vmlinux available, it's
> easy to assume you're tracing all calls to a function, when in fact some
> calls may be inlined within the same compilation unit.
> 
> A good example is tracing the rtnl_lock - there are multiple inlined
> copies, but neither bpftrace nor perf can warn you about it when debug
> info is absent.
> 

hi Jakub, see the RFC series at [1]. The goal is to represent inline
sites in BTF such that we can see when a function has been partially or
fully inlined, or indeed when optimizations have been applied to its ,
parameters which result in it being unsafe for fprobe()ing - in these
cases we skip representing such functions in BTF today.

In the case of inlined/optimized functions the proposal is to represent
them via BTF location data; not all of these locations will have all
parameters available due to optimization etc. However even absent that
it is still valuable to know such inlining has occurred as you say.

[1]
https://lore.kernel.org/bpf/20251008173512.731801-1-alan.maguire@oracle.com/

> $ sudo perf probe -a rtnl_lock
> Added new event:
>   probe:rtnl_lock      (on rtnl_lock)
>  
> You can now use it in all perf tools, such as:
>  
>         perf record -e probe:rtnl_lock -aR sleep 1
>  
> $ sudo apt install linux-image-`uname -r`-dbg
> Installing:
>   linux-image-6.12.53-cloudflare-2025.10.4-dbg
> […]
> $ sudo perf probe -d rtnl_lock
> Removed event: probe:rtnl_lock
> $ sudo perf probe -a rtnl_lock
> Added new events:
>   probe:rtnl_lock      (on rtnl_lock)
>   probe:rtnl_lock      (on rtnl_lock)
>   probe:rtnl_lock      (on rtnl_lock)
>   probe:rtnl_lock      (on rtnl_lock)
>   probe:rtnl_lock      (on rtnl_lock)
>   probe:rtnl_lock      (on rtnl_lock)
>   probe:rtnl_lock      (on rtnl_lock)
>   probe:rtnl_lock      (on rtnl_lock)
>   probe:rtnl_lock      (on rtnl_lock)
>   probe:rtnl_lock      (on rtnl_lock)
>   probe:rtnl_lock      (on rtnl_lock)
>  
> You can now use it in all perf tools, such as:
>  
>         perf record -e probe:rtnl_lock -aR sleep 1
>  
> $
> 
> Thanks,
> -jkbs