linux-kernel - Re: [PATCH] perf report: Fix no libunwind compiled warning break s390 issue

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Message-ID: <20200108143522.GE387467@krava>
Date:   Wed, 8 Jan 2020 15:35:22 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     "Jin, Yao" <yao.jin@...ux.intel.com>
Cc:     acme@...nel.org, jolsa@...nel.org, peterz@...radead.org,
        mingo@...hat.com, alexander.shishkin@...ux.intel.com,
        Linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        kan.liang@...el.com, yao.jin@...el.com, tmricht@...ux.ibm.com
Subject: Re: [PATCH] perf report: Fix no libunwind compiled warning break
 s390 issue

On Wed, Jan 08, 2020 at 09:02:02PM +0800, Jin, Yao wrote:
> 
> 
> On 1/8/2020 6:27 PM, Jiri Olsa wrote:
> > On Wed, Jan 08, 2020 at 03:17:45AM +0800, Jin Yao wrote:
> > > Commit 800d3f561659 ("perf report: Add warning when libunwind not compiled in")
> > > breaks the s390 platform. S390 uses libdw-dwarf-unwind for call chain
> > > unwinding and had no support for libunwind.
> > > 
> > > So the warning "Please install libunwind development packages during the perf build."
> > > caused the confusion even if the call-graph is displayed correctly.
> > > 
> > > This patch adds checking for HAVE_DWARF_SUPPORT, which is set when
> > > libdw-dwarf-unwind is compiled in.
> > > 
> > > Fixes: 800d3f561659 ("perf report: Add warning when libunwind not compiled in")
> > > 
> > > Reviewed-by: Thomas Richter <tmricht@...ux.ibm.com>
> > > Tested-by: Thomas Richter <tmricht@...ux.ibm.com>
> > > Signed-off-by: Jin Yao <yao.jin@...ux.intel.com>
> > 
> > perfect, I have the same change prepared for sending, but it's
> > together with making libdw default dwarf unwinder, which I'm still
> > not sure we want to do, so it all got posponed ;-)
> >  > would you guys be ok with that? with having libdw picked up as
> default dwarf unwinder..
> > 
> 
> I've roughly compared the performance between libunwind-dev and libdw-dev.
> While in my test (on KBL desktop), for the same perf report command-line, it
> looks the perf built with libunwind-dev is much faster than the perf built
> with libdw-dev.

ok, that's valid point.. the reason we start discussing it, was that
libunwind does not seem to support compressed ELF debug sections,
which works via libdw unwind

> 
> The command line is as following:
> 
> perf record --call-graph dwarf ./div
> perf report -g graph --stdio

I'll try to do some profiling and check with outr contact in libdw
to comment/check on that

thanks,
jirka