linux-kernel - Re: [patch] measurements, numbers about CONFIG_OPTIMIZE

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Date:	Fri, 9 Jan 2009 15:29:46 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Nicholas Miell <nmiell@...cast.net>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, jim owens <jowens@...com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	paulmck@...ux.vnet.ibm.com, Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>
Subject: Re: [patch] measurements, numbers about CONFIG_OPTIMIZE_INLINING=y
 impact


On Fri, 9 Jan 2009, Nicholas Miell wrote:

> On Fri, 2009-01-09 at 08:28 -0800, Linus Torvalds wrote:
> > 
> > We get oopses that have a nice symbolic back-trace, and it reports an 
> > error IN TOTALLY THE WRONG FUNCTION, because gcc "helpfully" inlined 
> > things to the point that only an expert can realize "oh, the bug was 
> > actually five hundred lines up, in that other function that was just 
> > called once, so gcc inlined it even though it is huge".
> > 
> > See? THIS is the problem with gcc heuristics. It's not about quality of 
> > code, it's about RELIABILITY of code. 
> 
> [bt]$ cat backtrace.c 
> #include <stdlib.h>
> 
> static void called_once()
> {
> 	abort();
> }
> 
> int main(int argc, char* argv[])
> {
> 	called_once();
> 	return 0;
> }
> [bt]$ gcc -Wall -O2 -g backtrace.c -o backtrace
> [bt]$ gdb --quiet backtrace
> (gdb) disassemble main 
> Dump of assembler code for function main:
> 0x00000000004004d0 <main+0>:	sub    $0x8,%rsp
> 0x00000000004004d4 <called_once+0>:	callq  0x4003b8 <abort@plt>
> End of assembler dump.
> (gdb) run
> Starting program: /home/nicholas/src/bitbucket/bt/backtrace 
> 
> Program received signal SIGABRT, Aborted.
> 0x0000003d9dc32f05 in raise (sig=<value optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
> 64	  return INLINE_SYSCALL (tgkill, 3, pid, selftid, sig);
> (gdb) bt
> #0  0x0000003d9dc32f05 in raise (sig=<value optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
> #1  0x0000003d9dc34a73 in abort () at abort.c:88
> #2  0x00000000004004d9 in called_once () at backtrace.c:5
> #3  main (argc=3989, argv=0xf95) at backtrace.c:10
> (gdb)
> 
> 
> Maybe the kernel's backtrace code should be fixed instead of blaming
> gcc.

Try doing the same without compiling with -g.

I believe Andi has a patch to use the DWARF markings for backtrace (I'm 
sure he'll correct me if I'm wrong ;-), but things like ftrace that use 
kallsyms to get the names of functions and such does better when the 
functions are not inlined. Not to mention that the function tracer does 
not trace inlined functions so that's another downside of inlining.

-- Steve
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