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

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Message-ID: <19016.8971.391008.394778@cargo.ozlabs.ibm.com>
Date:	Mon, 29 Jun 2009 12:12:27 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: performance counter ~0.4% error finding retired instruction
	count

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.

Paul.
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