linux-kernel - Re: [PATCH v3 07/19] unwind: Add user space unwinding API

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Message-ID: <20241209205458.vzwkh3ubch5sx5c7@jpoimboe>
Date: Mon, 9 Dec 2024 12:54:58 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Jens Remus <jremus@...ux.ibm.com>
Cc: x86@...nel.org, Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
	Sam James <sam@...too.org>, linux-trace-kernel@...r.kerne.org,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v3 07/19] unwind: Add user space unwinding API

On Fri, Dec 06, 2024 at 11:29:21AM +0100, Jens Remus wrote:
> On 28.10.2024 22:47, Josh Poimboeuf wrote:
> > +	if (ra == prev_ip)
> > +		goto the_end;
> 
> This seems too restrictive to me, as it effectively prevents
> unwinding from recursive functions, e.g. Glibc internal merge sort
> msort_with_tmp():
> 
> $ perf record -F 9999 --call-graph fp /usr/bin/objdump -wdWF /usr/bin/objdump
> $ perf script
> ...
> objdump    8314 236064.515562:     100010 task-clock:ppp:
>                  100630a compare_symbols+0x2a (/usr/bin/objdump)
>              3ffb9e58e7c msort_with_tmp.part.0+0x15c (/usr/lib64/libc.so.6)
>              3ffb9e58d76 msort_with_tmp.part.0+0x56 (/usr/lib64/libc.so.6)
> [unwinding unexpectedly stops]
> 
> Would it be an option to only stop unwinding if both the IP and SP do
> not change?
> 
> if (sp == prev_sp && ra == prev_ra)
> 	gote the_end;

Good point, I've already fixed that for the next version (not yet
posted).  I believe the only thing we really need to check here is that
the unwind is heading in the right direction:

if (cfa <= state->sp)
	goto the_end;

-- 
Josh