linux-kernel - Re: [PATCH/RFC v3] perf core: Allow setting up max frame stack depth via sysctl

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Date:	Wed, 27 Apr 2016 09:53:30 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Brendan Gregg <brendan.d.gregg@...il.com>,
	Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Alexei Starovoitov <ast@...nel.org>,
	He Kuang <hekuang@...wei.com>, Jiri Olsa <jolsa@...hat.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Milian Wolff <milian.wolff@...b.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Stephane Eranian <eranian@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	Wang Nan <wangnan0@...wei.com>, Zefan Li <lizefan@...wei.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC v3] perf core: Allow setting up max frame stack depth
 via sysctl

Em Tue, Apr 26, 2016 at 11:55:36PM +0200, Peter Zijlstra escreveu:
> On Tue, Apr 26, 2016 at 06:05:00PM -0300, Arnaldo Carvalho de Melo wrote:
> > > I started with max depth = 512, and even that was still truncated, and
> > > had to profile again at 1024 to capture the full stacks. Seems to
 
>                            ^^^^^^
 
> > And since you talked about userspace without frame pointers, have you
> > played with '--call-graph lbr'?
 
> That seems to be at odds with his requirements; he needs 1024 entries to
> capture full stacks, LBR is limited to 16/32 or so entries. That's 2
> orders of magnitude difference.

Duh, you're right, must've confused with that BTS thing, but as you said
in another message, that generates way too much info, have to read more
about it and the PT as well :-\

Anyway, for syscall backtraces, using 16 or 32 as the default in 'perf
trace', seems interesting, automatically switching to DWARF (or FP if
configured by the user in a system built with -fno-omit-frame-pointer)
if, say, --max-stack 38, say, is specified.

Have to take a look at how to do that for raw_syscalls:sys_enter.

- Arnaldo