[patch] kprobes: optimize branch placement

[Ingo Molnar]

* Ingo Molnar <mingo@xxxxxxx> wrote:

> * Ingo Molnar <mingo@xxxxxxx> 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@xxxxxxx>

optimize gcc's code generation by hinting branch probabilities.

Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
 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. */
-
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