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Date:   Fri, 29 Nov 2019 14:49:29 +0100
From:   Jiri Olsa <jolsa@redhat.com>
To:     Ivan Babrou <ivan@cloudflare.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        linux-perf-users@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>,
        kernel-team <kernel-team@cloudflare.com>
Subject: Re: perf is unable to read dward from go programs
Message-ID: <20191129134929.GA26903@krava>
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On Wed, Nov 27, 2019 at 01:15:20PM -0800, Ivan Babrou wrote:
> There were no response in linux-perf-users@, so I think it's fair to
> ask maintainers.
>=20
> On Fri, Nov 8, 2019 at 3:53 PM Ivan Babrou <ivan@cloudflare.com> wrote:
> >
> > I have a simple piece of code that burns CPU for 1s:
> >
> > * https://gist.github.com/bobrik/cf022ff6950d09032fa13a984e2272ed
> >
> > I can build it just fine: go build -o /tmp/burn burn.go
> >
> > And I can see correct stacks if I record with fp:
> >
> > perf record -e cpu-clock -g -F 99 /tmp/burn
> >
> > But if I record with gwarf:
> >
> > perf record -e cpu-clock -g -F 99 --call-graph dwarf /tmp/burn
> >
> > Then stacks are lost with the following complaints during "perf script"=
:
> >
> > BFD: Dwarf Error: found dwarf version '376', this reader only handles
> > version 2, 3 and 4 information.
> > BFD: Dwarf Error: found dwarf version '31863', this reader only
> > handles version 2, 3 and 4 information.
> > BFD: Dwarf Error: found dwarf version '65271', this reader only
> > handles version 2, 3 and 4 information.
> > BFD: Dwarf Error: found dwarf version '289', this reader only handles
> > version 2, 3 and 4 information.

hi,
the binary generated by go has compressed debug info (on my setup)
and libunwind (default dwarf unwinder) does not seem to support that

but when I compile perf with libdw unwind support:

  $ make DEBUG=3D1 VF=3D1 NO_LIBUNWIND=3D1

I'm getting proper backtraces (below), maybe it's time to change
the default dwarf unwinder ;-)

thanks,
jirka


---
    51.63%  ex       ex                [.] crypto/sha512.blockAVX2
            |
            ---crypto/sha512.blockAVX2
               |         =20
                --51.48%--crypto/sha512.block
                          crypto/sha512.(*digest).Write
                          crypto/sha512.(*digest).checkSum
                          crypto/sha512.(*digest).Sum
                          main.burn
                          main.main
                          runtime.main
                          runtime.goexit

    11.55%  ex       ex                [.] runtime.mallocgc
            |
            ---runtime.mallocgc
               |         =20
               |--7.45%--runtime.newobject
               |          |         =20
               |           --7.45%--main.burn
               |                     main.main
               |                     runtime.main
               |                     runtime.goexit
               |         =20
                --3.40%--runtime.growslice
                          crypto/sha512.(*digest).Sum
                          main.burn
                          main.main
                          runtime.main
                          runtime.goexit

     3.69%  ex       ex                [.] crypto/sha512.(*digest).Write
            |
            ---crypto/sha512.(*digest).Write
               |         =20
               |--2.91%--crypto/sha512.(*digest).checkSum
               |          crypto/sha512.(*digest).Sum
               |          main.burn
               |          main.main
               |          runtime.main
               |          runtime.goexit
               |         =20
                --0.57%--main.burn
                          main.main
                          runtime.main
                          runtime.goexit

     3.44%  ex       ex                [.] runtime.memclrNoHeapPointers
            |
            ---runtime.memclrNoHeapPointers
               |         =20
                --2.92%--runtime.(*mheap).alloc
                          runtime.(*mcentral).grow
                          runtime.(*mcentral).cacheSpan
                          runtime.(*mcache).refill
                          runtime.(*mcache).nextFree
                          runtime.mallocgc
                          |         =20
                          |--2.27%--runtime.newobject
                          |          main.burn
                          |          main.main
                          |          runtime.main
                          |          runtime.goexit
                          |         =20
                           --0.64%--runtime.growslice
                                     crypto/sha512.(*digest).Sum
                                     main.burn
                                     main.main
                                     runtime.main
                                     runtime.goexit
...