Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

Ingo Molnar
2006-09-16T19:45:52Z

* 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.

Ingo

--------------->
Subject: [patch] kprobes: speed INT3 trap handling up on i386
From: Ingo Molnar <mingo@elte.hu>

speed up kprobes trap handling by special-casing kernel-space
INT3 traps (which do not occur otherwise) and doing a kprobes
handler check - instead of redirecting over the i386-die-notifier
chain.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
arch/i386/kernel/kprobes.c | 2 +-
arch/i386/kernel/traps.c | 19 ++++++++++++-------
include/asm-i386/kprobes.h | 2 ++
3 files changed, 15 insertions(+), 8 deletions(-)

Index: linux/arch/i386/kernel/kprobes.c
===================================================================
--- linux.orig/arch/i386/kernel/kprobes.c
+++ linux/arch/i386/kernel/kprobes.c
@@ -200,7 +200,7 @@ void __kprobes arch_prepare_kretprobe(st
* Interrupts are disabled on entry as trap3 is an interrupt gate and they
* remain disabled thorough out this function.
*/
-static int __kprobes kprobe_handler(struct pt_regs *regs)
+int __kprobes kprobe_handler(struct pt_regs *regs)
{
struct kprobe *p;
int ret = 0;
Index: linux/arch/i386/kernel/traps.c
===================================================================
--- linux.orig/arch/i386/kernel/traps.c
+++ linux/arch/i386/kernel/traps.c
@@ -802,13 +802,18 @@ EXPORT_SYMBOL_GPL(unset_nmi_callback);
#ifdef CONFIG_KPROBES
fastcall void __kprobes do_int3(struct pt_regs *regs, long error_code)
{
- if (notify_die(DIE_INT3, "int3", regs, error_code, 3, SIGTRAP)
- == NOTIFY_STOP)
- return;
- /* This is an interrupt gate, because kprobes wants interrupts
- disabled. Normal trap handlers don't. */
- restore_interrupts(regs);
- do_trap(3, SIGTRAP, "int3", 1, regs, error_code, NULL);
+ /*
+ * kernel-mode INT3s are likely kprobes:
+ */
+ if (!user_mode(regs)) {
+ if (kprobe_handler(regs))
+ return;
+ /* This is an interrupt gate, because kprobes wants interrupts
+ disabled. Normal trap handlers don't. */
+ restore_interrupts(regs);
+ do_trap(3, SIGTRAP, "int3", 1, regs, error_code, NULL);
+ }
+ notify_die(DIE_INT3, "int3", regs, error_code, 3, SIGTRAP);
}
#endif

Index: linux/include/asm-i386/kprobes.h
===================================================================
--- linux.orig/include/asm-i386/kprobes.h
+++ linux/include/asm-i386/kprobes.h
@@ -88,4 +88,6 @@ static inline void restore_interrupts(st

extern int kprobe_exceptions_notify(struct notifier_block *self,
unsigned long val, void *data);
+extern int kprobe_handler(struct pt_regs *regs);
+
#endif /* _ASM_KPROBES_H */