Re: [PATCH 0/2] Better error messages for 'perf probe --add'

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 16 Apr 2024 12:46:23 -0300
Arnaldo Carvalho de Melo <acme@kernel.org> wrote:

> On Mon, Apr 15, 2024 at 09:55:09PM -0700, Dima Kogan wrote:
> > Hi.
> > 
> > I tried to add some userspace probes to some C++ code, and got this:
> > 
> >   $ sudo perf probe -x tst --add _ZN.....
> >   Error: Failed to add events.
> 
> Both patches look great, Masami, can I have your Acked-by?
> 
> If you find other cases where the error messages can be improved, feel
> free to send us, please CC Masami, as I'm doing now.

Yeah, looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thank you,

> 
> Thanks!
> 
> - Rrnaldo
>  
> > Note the completely non-useful error message. The issue is that the C++ symbol
> > mangling can create very loooong symbol names, causing perf to try to use very
> > long probe names by default, causing this problem. It took a little while to
> > figure this out, and a better error message would have helped, hence these
> > patches.
> > 
> > To reproduce, here's a tst.cc:
> > 
> >   #include <stdio.h>
> > 
> >   namespace n1111111111111111111111111111111111111111111111111111111111111111 {
> >       namespace n2222222222222222222222222222222222222222222222222222222222222222 {
> > 
> >           void f(void)
> >           {
> >               printf("f()\n");
> >           }
> >       }
> >   }
> > 
> >   int main(void)
> >   {
> >       n1111111111111111111111111111111111111111111111111111111111111111::n2222222222222222222222222222222222222222222222222222222222222222::f();
> >       n1111111111111111111111111111111111111111111111111111111111111111::n2222222222222222222222222222222222222222222222222222222222222222::f();
> >       return 0;
> >   }
> > 
> > I then
> > 
> >   g++ -g -o tst tst.cc
> > 
> > and
> > 
> >   sudo perf probe -x ~/tmp/tst --add _ZN65n111111111111111111111111111111111111111111111111111111111111111165n22222222222222222222222222222222222222222222222222222222222222221fEv
> > 
> > After the patch a better error message results, and the workaround is clear:
> > 
> >   sudo perf probe -x ~/tmp/tst --add probe=_ZN65n111111111111111111111111111111111111111111111111111111111111111165n22222222222222222222222222222222222222222222222222222222222222221fEv
> > 
> > Dima Kogan (2):
> >   perf probe-event: un-hardcoded sizeof(buf)
> >   perf probe-event: better error message for a too-long probe name
> > 
> >  tools/perf/util/probe-event.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > -- 
> > 2.42.0
> > 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>