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From: Milian Wolff <milian.wolff@kdab.com>
To: Jin Yao <yao.jin@linux.intel.com>
Cc: acme@kernel.org, jolsa@kernel.org, Linux-kernel@vger.kernel.org,
        ak@linux.intel.com, kan.liang@intel.com, yao.jin@intel.com
Subject: Re: [PATCH v5 0/5] perf report: Show inline stack
Date: Sat, 18 Mar 2017 17:41:09 +0100
Message-ID: <1532086.vKWRnURGfG@agathebauer>
Organization: KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
In-Reply-To: <1489700547-7260-1-git-send-email-yao.jin@linux.intel.com>
References: <1489700547-7260-1-git-send-email-yao.jin@linux.intel.com>
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On Donnerstag, 16. M=E4rz 2017 22:42:22 CET Jin Yao wrote:
> v5: Update according to Milian Wolff's comments. It groups by address
>     (then display file/ line), or by function (then display function name=
).

Thank you Jin, that is really good. I tested it and it works really well fo=
r=20
me.

Arnaldo, could you please consider merging this? It's an extremely useful=20
feature and direly missing from perf so far.

That said, Jin, here are some observations that could be improved in the=20
future (I don't think any of these should hold back merging this feature no=
w):

=46or the following example code build with "-O2 -g" and recorded with "--c=
all-
graph dwarf" I observe some output combinations that could potentially be=20
improved in the future:

~~~~~~~~~~~~~~~~~~~~
#include <complex>
#include <cmath>
#include <random>
#include <iostream>

using namespace std;

int main()
{
    uniform_real_distribution<double> uniform(-1E5, 1E5);
    default_random_engine engine;
    double s =3D 0;
    for (int i =3D 0; i < 10000000; ++i) {
        s +=3D norm(complex<double>(uniform(engine), uniform(engine)));
    }
    cout << s << '\n';
    return 0;
}
~~~~~~~~~~~~~~~~

#1 duplicated entries when grouping by function:

~~~~~~~~~~~~~~~~
perf report --inline --stdio
=2E..
             --35.34%--_start
                       __libc_start_main
                       main
                       main (inline)
                       std::uniform_real_distribution<double>::operator()<s=
td::linear_congruential_engine<unsigned=20
long, 16807ul, 0ul, 2147483647ul> > (inline)
                       std::uniform_real_distribution<double>::operator()<s=
td::linear_congruential_engine<unsigned=20
long, 16807ul, 0ul, 2147483647ul> > (inline)
                       std::__detail::_Adaptor<std::linear_congruential_eng=
ine<unsigned=20
long, 16807ul, 0ul, 2147483647ul>, double>::operator() (inline)
~~~~~~~~~~~~~~~~

Here, we see main twice, once for the "real" frame, and once for an inlined=
=20
one? Then we see the same function twice as inlined frame, which is also od=
d.

~~~~~~~~~~~~~~~~
perf report --inline --stdio --no-children
=2E..
    59.81%  cpp-inlining  libm-2.25.so      [.] __hypot_finite
            |
            ---__hypot_finite
               hypot
               main
               std::norm<double> (inline)
               main (inline)
               __libc_start_main
               _start
~~~~~~~~~~~~~~~~

Here we see a confusing output. The first "main" frame below "hypot" is=20
actually code form cpp's complex header which got inlined into main. That=20
associates the wrong function name to this frame, i.e. "main" instead of=20
std::norm". When the inline stack is shown below we actually see what happe=
ns,=20
i.e. we eventually end up in main again, but of course this output is not t=
he=20
best as-is.

But, again: I think these are minor issues, and the feature itself is alrea=
dy=20
extremely useful and I hope to see it finally merged.

Thanks again Jin for your good work!

Cheers

=2D-=20
Milian Wolff | milian.wolff@kdab.com | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts
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