[RFC][ PATCH -tip 0/6] kprobes: Kprobes jump optimization support

From: Masami Hiramatsu
Date: Fri Jun 12 2009 - 18:48:50 EST


Here are the RFC patchset of the kprobes jump optimization
(a.k.a. Djprobe, which I had developed two years ago). This version
supports all features of Kprobes (probe disabling, module .init probing,
debugfs interface).

These patches can be applied on -tip tree + x86 instruction decoder
which I re-sent right now (see below). This is another example of
x86 instruction decoder.

This patch depends on:
kprobes: cleanup fix_riprel() using insn decoder on x86
kprobes: checks probe address is instruction boudary on x86
x86: x86 instruction decoder build-time selftest
x86: instruction decoder API

Jump Optimized Kprobes
o Concept
Kprobes uses the int3 breakpoint instruction on x86 for instrumenting
probes into running kernel. Jump optimization allows kprobes to replace
breakpoint with a jump instruction for reducing probing overhead drastically.

o Performance
An optimized kprobe 5 times faster than a kprobe.

Optimizing probes gains its performance. Usually, a kprobe hit takes
0.5 to 1.0 microseconds to process. On the other hand, a jump optimized
probe hit takes less than 0.1 microseconds (actual number depends on the
processor). Here is a sample overheads.

Intel(R) Xeon(R) CPU E5410 @ 2.33GHz (without debugging options)

x86-32 x86-64
kprobe: 0.68us 0.91us
kprobe+booster: 0.27us 0.40us
kprobe+optimized: 0.06us 0.06us

kretprobe : 0.95us 1.21us
kretprobe+booster: 0.53us 0.71us
kretprobe+optimized: 0.30us 0.35us

(booster skips single-stepping)

Note that jump optimization also consumes more memory, but not so much.
It just uses ~200 bytes, so, even if you use ~10,000 probes, it just
consumes a few MB.

o Usage
Set CONFIG_OPTPROBES=y when building a kernel, then all *probes will be
optimized if possible.

Kprobes decodes probed function and checks whether the target instructions
can be optimized(replaced with a jump) safely. If it can't be, Kprobes just
doesn't optimize it.

o Optimization
Before preparing optimization, Kprobes inserts original(user-defined)
kprobe on the specified address. So, even if the kprobe is not
possible to be optimized, it just uses a normal kprobe.

- Safety check
First, Kprobes gets the address of probed function and checks whether the
optimized region, which will be replaced by a jump instruction, does NOT
straddle the function boundary, because if the optimized region reaches the
next function, its caller causes unexpected results.
Next, Kprobes decodes whole body of probed function and checks there is
NO indirect jump, and near jump which jumps into the optimized region (except
the 1st byte of jump), because if some jump instruction jumps into the middle
of another instruction, it causes unexpected results too.
Kprobes also measures the length of instructions which will be replaced
by a jump instruction, because a jump instruction is longer than 1 byte,
it may replaces multiple instructions, and it checks whether those
instructions can be executed out-of-line.

- Preparing detour code
Then, Kprobes prepares "detour" buffer, which contains exception emulating
code (push/pop registers, call handler), copied instructions(Kprobes copies
instructions which will be replaced by a jump, to the detour buffer), and
a jump which jumps back to the original execution path.

- Pre-optimization
After preparing detour code, Kprobes enqueues the kprobe to optimizing list
and kicks kprobe-optimizer workqueue to optimize it. To wait other optimized
probes, kprobe-optimizer will delay to work.
When the optimized-kprobe is hit before optimization, its handler
changes IP(instruction pointer) to copied code and exits. So, the
instructions which were copied to detour buffer are executed on the detour

- Optimization
Kprobe-optimizer doesn't start instruction-replacing soon, it waits
synchronize_sched for safety, because some processors are possible to be
interrupted on the instructions which will be replaced by a jump instruction.
As you know, synchronize_sched() can ensure that all interruptions which were
executed when synchronize_sched() was called are done, only if
CONFIG_PREEMPT=n. So, this version supports only the kernel with
After that, kprobe-optimizer replaces the 4 bytes right after int3 breakpoint
with relative-jump destination, and synchronize caches on all processors. Next,
it replaces int3 with relative-jump opcode, and synchronize caches again.

- Unoptimization
When unregistering, disabling kprobe or being blocked by other kprobe,
an optimized-kprobe will be unoptimized. Before kprobe-optimizer runs,
the kprobe just be dequeued from the optimized list. When the optimization
has been done, it replaces a jump with int3 breakpoint and original code.
First it puts int3 at the first byte of the jump, synchronize caches
on all processors, and replaces the 4 bytes right after int3 with the
original code.

(*)This optimization-safety checking may be replaced with stop-machine method
which ksplice is done for supporting CONFIG_PREEMPT=y kernel.

Thank you,


Masami Hiramatsu (6):
kprobes: add documents of jump optimization
kprobes: x86: support kprobes jump optimization on x86
kprobes: x86: cleanup save/restore registers
kprobes: kprobes jump optimization core
kprobes: introducing generic insn_slot framework
kprobes: use list instead of hlist for insn_pages

Documentation/kprobes.txt | 172 ++++++++++++-
arch/Kconfig | 11 +
arch/x86/Kconfig | 1
arch/x86/include/asm/kprobes.h | 31 ++
arch/x86/kernel/kprobes.c | 528 ++++++++++++++++++++++++++++++++++------
include/linux/kprobes.h | 38 +++
kernel/kprobes.c | 508 +++++++++++++++++++++++++++++++-------
7 files changed, 1096 insertions(+), 193 deletions(-)

Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@xxxxxxxxxx
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