[patch] kprobes: optimize branch placement

Ingo Molnar
2006-09-16T20:38:10Z

* Ingo Molnar <mingo@elte.hu> wrote:

> * Ingo Molnar <mingo@elte.hu> wrote:
>
> > and have measured the overhead of an unmodified, kprobes-probed and
> > djprobes-probed sys_getpid() system-call:
> >
> > sys_getpid() unmodified latency: 317 cycles [ 0.146 usecs ]
> > sys_getpid() kprobes latency: 815 cycles [ 0.377 usecs ]
> > sys_getpid() djprobes latency: 380 cycles [ 0.176 usecs ]
>
> i have taken a look at the kprobes fastpath, and there are a few things
> we can do to speed it up. The patch below shaves off 75 cycles from the
> kprobes overhead:
>
> sys_getpid() kprobes-speedup: 740 cycles [ 0.342 usecs ]
>
> that reduces the kprobes overhead to 423 cycles.

the patch below brings the overhead down to 420 cycles:

sys_getpid() kprobes-speedup: 737 cycles [ 0.341 usecs ]

Ingo

---------->
Subject: [patch] kprobes: optimize branch placement
From: Ingo Molnar <mingo@elte.hu>

optimize gcc's code generation by hinting branch probabilities.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
arch/i386/kernel/kprobes.c | 2 +-
arch/i386/kernel/traps.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)

Index: linux/arch/i386/kernel/kprobes.c
===================================================================
--- linux.orig/arch/i386/kernel/kprobes.c
+++ linux/arch/i386/kernel/kprobes.c
@@ -220,7 +220,7 @@ int __kprobes kprobe_handler(struct pt_r
kcb = get_kprobe_ctlblk();

/* Check we're not actually recursing */
- if (kprobe_running()) {
+ if (unlikely(kprobe_running())) {
p = get_kprobe(addr);
if (p) {
if (kcb->kprobe_status == KPROBE_HIT_SS &&
Index: linux/arch/i386/kernel/traps.c
===================================================================
--- linux.orig/arch/i386/kernel/traps.c
+++ linux/arch/i386/kernel/traps.c
@@ -806,7 +806,7 @@ fastcall void __kprobes do_int3(struct p
* kernel-mode INT3s are likely kprobes:
*/
if (!user_mode(regs)) {
- if (kprobe_handler(regs))
+ if (likely(kprobe_handler(regs)))
return;
/* This is an interrupt gate, because kprobes wants interrupts
disabled. Normal trap handlers don't. */