linux-kernel - Re: [PATCH] Kprobes: Add kprobes smoke tests that run on boot

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Message-Id: <1199910240.4162.31.camel@dyn9047018096.beaverton.ibm.com>
Date:	Wed, 09 Jan 2008 12:24:00 -0800
From:	Jim Keniston <jkenisto@...ibm.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	ananth@...ibm.com, akpm@...ux-foundation.org,
	lkml <linux-kernel@...r.kernel.org>, mingo@...e.hu,
	mhiramat@...hat.com, davem@...emloft.net, hskinnemoen@...el.com
Subject: Re: [PATCH] Kprobes: Add kprobes smoke tests that run on boot

On Wed, 2008-01-09 at 00:28 +0100, Andi Kleen wrote:
> > I have no problem with that, but if we want to make it buildable as a
> > module, the call to get_kprobe() needs to be replaced with some other
> > gcc-inline-defeating mechanism, or we need to export get_probe().  I
> 
> It's still unclear where noinline does not work (do you have details on that? 
> It sounds a little dubious) ...

Here's a simplified version of a module we've used to gauge kprobes
performance.  On my system (i386, 2.6.23-rc8 at the moment, gcc version
4.1.2), the module calls the (empty) probed function (probeme) niter
times, but reports zero probe hits -- indicating probeme was inlined
even though it was declared noinline.

If I change probeme to a varargs function, per your suggestion -- see
diffs below -- the proper number of probe hits are reported.
----- noinline.c -----
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/jiffies.h>
#include <linux/kprobes.h>

/*
 * Sample usage: insmod noinline.ko niter=5000000
 * printk should report 5000000 calls.
 */

static int niter = 0;
module_param(niter, int, 0);
MODULE_PARM_DESC(niter, "iterations");

static int ncalls = 0;

static int counter_hdlr(struct kprobe *p, struct pt_regs *regs)
{
	ncalls++;
	return 0;
}

// #define noinline __attribute__((noinline))
noinline static void probeme(void)
{
}

struct kprobe kp = {
	.addr = (kprobe_opcode_t*) probeme,
	.pre_handler = counter_hdlr
};

static int __init noinline_init(void)
{
	int ret;
	int i;
	u64 start, stop;

	ret = register_kprobe(&kp);
	if (ret != 0) {
		printk(KERN_ERR "register_kprobe returns %d\n", ret);
		return ret;
	}

	start = get_jiffies_64();
	for (i = 1; i <= niter; i++)
		probeme();
	stop = get_jiffies_64();
	printk(KERN_INFO "noinline test: %d iterations in %lld jiffies\n",
		niter, stop-start);
	printk(KERN_INFO "Counted %d calls\n", ncalls);
	return 0;
}

static void __exit noinline_exit(void)
{
	unregister_kprobe(&kp);
	printk(KERN_INFO "noinline test: exiting...\n");
}

module_init(noinline_init);
module_exit(noinline_exit);
MODULE_LICENSE("GPL");
----- Change probeme to a varargs function -----
--- noinline.c	2008-01-09 11:26:09.000000000 -0800
+++ noinline2.c	2008-01-09 11:59:54.000000000 -0800
@@ -22,8 +22,13 @@
 }
 
 // #define noinline __attribute__((noinline))
-noinline static void probeme(void)
+noinline static void probeme(int nargs, ...)
 {
+	va_list args;
+	va_start(args, nargs);
+	while (nargs-- > 0)
+		printk(KERN_INFO "%p\n", va_arg(args, char*));
+	va_end(args);
 }
 
 struct kprobe kp = {
@@ -45,9 +50,9 @@
 
 	start = get_jiffies_64();
 	for (i = 1; i <= niter; i++)
-		probeme();
+		probeme(0);
 	stop = get_jiffies_64();
-	printk(KERN_INFO "noinline test: %d iterations in %lld jiffies\n",
+	printk(KERN_INFO "noinline2 test: %d iterations in %lld jiffies\n",
 		niter, stop-start);
 	printk(KERN_INFO "Counted %d calls\n", ncalls);
 	return 0;
@@ -56,7 +61,7 @@
 static void __exit noinline_exit(void)
 {
 	unregister_kprobe(&kp);
-	printk(KERN_INFO "noinline test: exiting...\n");
+	printk(KERN_INFO "noinline2 test: exiting...\n");
 }
 
 module_init(noinline_init);

...
> -Andi

Thanks for the suggestions.
Jim

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