linux-kernel - Re: performance counter ~0.4% error finding retired instruction count

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Date:	Mon, 29 Jun 2009 05:48:40 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: performance counter ~0.4% error finding retired instruction
	count


* Paul Mackerras <paulus@...ba.org> wrote:

> I can think of three ways to eliminate the PLT resolver overhead on
> execvp:
> 
> (1) Do execvp on a non-executable file first to get execvp resolved:
> 
> 	char tmpnam[16];
> 	int fd;
> 	char *args[1];
> 
> 	strcpy(tmpname, "/tmp/perfXXXXXX");
> 	fd = mkstemp(tmpname);
> 	if (fd >= 0) {
> 		args[1] = NULL;
> 		execvp(tmpname, args);
> 		close(fd);
> 		unlink(tmpname);
> 	}
> 	enable_counters();
> 	execvp(prog, argv);
> 
> (2) Look up execvp in glibc and call it directly:
> 
> 	int (*execptr)(const char *, char *const []);
> 
> 	execptr = dlsym(RTLD_NEXT, "execvp");
> 	enable_counters();
> 	(*execptr)(prog, argv);
> 
> (3) Resolve the executable path ourselves and then invoke the execve
> system call directly:
> 
> 	char *execpath;
> 
> 	execpath = search_path(getenv("PATH"), prog);
> 	enable_counters();
> 	syscall(NR_execve, execpath, argv, envp);
> 
> (4) Same as (1), but rely on "" being an invalid program name for
> execvp:
> 
> 	execvp("", argv);
> 	enable_counters();
> 	execvp(prog, argv);
> 
> What do you guys think?  Does any of these appeal more than the
> others?  I'm leaning towards (4) myself.

(4) looks convincingly elegant.

We could also do (5): a one-shot counters-disabled ptrace run of the 
target, then enable-counters-in-target + ptrace-detach after the 
first stop.

	Ingo
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