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kprobes: Add documents of jump optimization

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Add documentations about kprobe jump optimization to
Documentation/kprobes.txt.

Changes in v10:
 - Editorial fixups by Jim Keniston.

Changes in v8:
 - Update documentation and benchmark results.

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Signed-off-by: Jim Keniston <jkenisto@us.ibm.com>
Cc: systemtap <systemtap@sources.redhat.com>
Cc: DLE <dle-develop@lists.sourceforge.net>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Anders Kaseorg <andersk@ksplice.com>
Cc: Tim Abbott <tabbott@ksplice.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Jason Baron <jbaron@redhat.com>
Cc: Mathieu Desnoyers <compudj@krystal.dyndns.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
LKML-Reference: <20100225133504.6725.79395.stgit@localhost6.localdomain6>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Masami Hiramatsu 2010-02-25 08:35:04 -05:00 committed by Ingo Molnar
parent c0f7ac3a9e
commit b26486bf75
1 changed files with 195 additions and 12 deletions
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207
Documentation/kprobes.txt
View file

@ -1,6 +1,7 @@
Title : Kernel Probes (Kprobes)
Authors : Jim Keniston <jkenisto@us.ibm.com>
: Prasanna S Panchamukhi <prasanna@in.ibm.com>
: Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com>
: Masami Hiramatsu <mhiramat@redhat.com>
CONTENTS
@ -15,6 +16,7 @@ CONTENTS
9. Jprobes Example
10. Kretprobes Example
Appendix A: The kprobes debugfs interface
Appendix B: The kprobes sysctl interface
1. Concepts: Kprobes, Jprobes, Return Probes
@ -42,13 +44,13 @@ registration/unregistration of a group of *probes. These functions
can speed up unregistration process when you have to unregister
a lot of probes at once.
The next three subsections explain how the different types of
probes work. They explain certain things that you'll need to
know in order to make the best use of Kprobes -- e.g., the
difference between a pre_handler and a post_handler, and how
to use the maxactive and nmissed fields of a kretprobe. But
if you're in a hurry to start using Kprobes, you can skip ahead
to section 2.
The next four subsections explain how the different types of
probes work and how jump optimization works. They explain certain
things that you'll need to know in order to make the best use of
Kprobes -- e.g., the difference between a pre_handler and
a post_handler, and how to use the maxactive and nmissed fields of
a kretprobe. But if you're in a hurry to start using Kprobes, you
can skip ahead to section 2.
1.1 How Does a Kprobe Work?
@ -161,13 +163,125 @@ In case probed function is entered but there is no kretprobe_instance
object available, then in addition to incrementing the nmissed count,
the user entry_handler invocation is also skipped.
1.4 How Does Jump Optimization Work?
If you configured your kernel with CONFIG_OPTPROBES=y (currently
this option is supported on x86/x86-64, non-preemptive kernel) and
the "debug.kprobes_optimization" kernel parameter is set to 1 (see
sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
instruction instead of a breakpoint instruction at each probepoint.
1.4.1 Init a Kprobe
When a probe is registered, before attempting this optimization,
Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
address. So, even if it's not possible to optimize this particular
probepoint, there'll be a probe there.
1.4.2 Safety Check
Before optimizing a probe, Kprobes performs the following safety checks:
- Kprobes verifies that the region that will be replaced by the jump
instruction (the "optimized region") lies entirely within one function.
(A jump instruction is multiple bytes, and so may overlay multiple
instructions.)
- Kprobes analyzes the entire function and verifies that there is no
jump into the optimized region. Specifically:
- the function contains no indirect jump;
- the function contains no instruction that causes an exception (since
the fixup code triggered by the exception could jump back into the
optimized region -- Kprobes checks the exception tables to verify this);
and
- there is no near jump to the optimized region (other than to the first
byte).
- For each instruction in the optimized region, Kprobes verifies that
the instruction can be executed out of line.
1.4.3 Preparing Detour Buffer
Next, Kprobes prepares a "detour" buffer, which contains the following
instruction sequence:
- code to push the CPU's registers (emulating a breakpoint trap)
- a call to the trampoline code which calls user's probe handlers.
- code to restore registers
- the instructions from the optimized region
- a jump back to the original execution path.
1.4.4 Pre-optimization
After preparing the detour buffer, Kprobes verifies that none of the
following situations exist:
- The probe has either a break_handler (i.e., it's a jprobe) or a
post_handler.
- Other instructions in the optimized region are probed.
- The probe is disabled.
In any of the above cases, Kprobes won't start optimizing the probe.
Since these are temporary situations, Kprobes tries to start
optimizing it again if the situation is changed.
If the kprobe can be optimized, Kprobes enqueues the kprobe to an
optimizing list, and kicks the kprobe-optimizer workqueue to optimize
it. If the to-be-optimized probepoint is hit before being optimized,
Kprobes returns control to the original instruction path by setting
the CPU's instruction pointer to the copied code in the detour buffer
-- thus at least avoiding the single-step.
1.4.5 Optimization
The Kprobe-optimizer doesn't insert the jump instruction immediately;
rather, it calls synchronize_sched() for safety first, because it's
possible for a CPU to be interrupted in the middle of executing the
optimized region(*). As you know, synchronize_sched() can ensure
that all interruptions that were active when synchronize_sched()
was called are done, but only if CONFIG_PREEMPT=n. So, this version
of kprobe optimization supports only kernels with CONFIG_PREEMPT=n.(**)
After that, the Kprobe-optimizer calls stop_machine() to replace
the optimized region with a jump instruction to the detour buffer,
using text_poke_smp().
1.4.6 Unoptimization
When an optimized kprobe is unregistered, disabled, or blocked by
another kprobe, it will be unoptimized. If this happens before
the optimization is complete, the kprobe is just dequeued from the
optimized list. If the optimization has been done, the jump is
replaced with the original code (except for an int3 breakpoint in
the first byte) by using text_poke_smp().
(*)Please imagine that the 2nd instruction is interrupted and then
the optimizer replaces the 2nd instruction with the jump *address*
while the interrupt handler is running. When the interrupt
returns to original address, there is no valid instruction,
and it causes an unexpected result.
(**)This optimization-safety checking may be replaced with the
stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
kernel.
NOTE for geeks:
The jump optimization changes the kprobe's pre_handler behavior.
Without optimization, the pre_handler can change the kernel's execution
path by changing regs->ip and returning 1. However, when the probe
is optimized, that modification is ignored. Thus, if you want to
tweak the kernel's execution path, you need to suppress optimization,
using one of the following techniques:
- Specify an empty function for the kprobe's post_handler or break_handler.
or
- Config CONFIG_OPTPROBES=n.
or
- Execute 'sysctl -w debug.kprobes_optimization=n'
2. Architectures Supported
Kprobes, jprobes, and return probes are implemented on the following
architectures:
- i386
- x86_64 (AMD-64, EM64T)
- i386 (Supports jump optimization)
- x86_64 (AMD-64, EM64T) (Supports jump optimization)
- ppc64
- ia64 (Does not support probes on instruction slot1.)
- sparc64 (Return probes not yet implemented.)
@ -193,6 +307,10 @@ it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
so you can use "objdump -d -l vmlinux" to see the source-to-object
code mapping.
If you want to reduce probing overhead, set "Kprobes jump optimization
support" (CONFIG_OPTPROBES) to "y". You can find this option under the
"Kprobes" line.
4. API Reference
The Kprobes API includes a "register" function and an "unregister"
@ -389,7 +507,10 @@ the probe which has been registered.
Kprobes allows multiple probes at the same address. Currently,
however, there cannot be multiple jprobes on the same function at
the same time.
the same time. Also, a probepoint for which there is a jprobe or
a post_handler cannot be optimized. So if you install a jprobe,
or a kprobe with a post_handler, at an optimized probepoint, the
probepoint will be unoptimized automatically.
In general, you can install a probe anywhere in the kernel.
In particular, you can probe interrupt handlers. Known exceptions
@ -453,6 +574,38 @@ reason, Kprobes doesn't support return probes (or kprobes or jprobes)
on the x86_64 version of __switch_to(); the registration functions
return -EINVAL.
On x86/x86-64, since the Jump Optimization of Kprobes modifies
instructions widely, there are some limitations to optimization. To
explain it, we introduce some terminology. Imagine a 3-instruction
sequence consisting of a two 2-byte instructions and one 3-byte
instruction.
IA
|
[-2][-1][0][1][2][3][4][5][6][7]
[ins1][ins2][ ins3 ]
[<- DCR ->]
[<- JTPR ->]
ins1: 1st Instruction
ins2: 2nd Instruction
ins3: 3rd Instruction
IA: Insertion Address
JTPR: Jump Target Prohibition Region
DCR: Detoured Code Region
The instructions in DCR are copied to the out-of-line buffer
of the kprobe, because the bytes in DCR are replaced by
a 5-byte jump instruction. So there are several limitations.
a) The instructions in DCR must be relocatable.
b) The instructions in DCR must not include a call instruction.
c) JTPR must not be targeted by any jump or call instruction.
d) DCR must not straddle the border betweeen functions.
Anyway, these limitations are checked by the in-kernel instruction
decoder, so you don't need to worry about that.
6. Probe Overhead
On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
@ -476,6 +629,19 @@ k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
6.1 Optimized Probe Overhead
Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
process. Here are sample overhead figures (in usec) for x86 architectures.
k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
7. TODO
a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
@ -523,7 +689,8 @@ is also specified. Following columns show probe status. If the probe is on
a virtual address that is no longer valid (module init sections, module
virtual addresses that correspond to modules that've been unloaded),
such probes are marked with [GONE]. If the probe is temporarily disabled,
such probes are marked with [DISABLED].
such probes are marked with [DISABLED]. If the probe is optimized, it is
marked with [OPTIMIZED].
/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
@ -533,3 +700,19 @@ registered probes will be disarmed, till such time a "1" is echoed to this
file. Note that this knob just disarms and arms all kprobes and doesn't
change each probe's disabling state. This means that disabled kprobes (marked
[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
Appendix B: The kprobes sysctl interface
/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
a knob to globally and forcibly turn jump optimization (see section
1.4) ON or OFF. By default, jump optimization is allowed (ON).
If you echo "0" to this file or set "debug.kprobes_optimization" to
0 via sysctl, all optimized probes will be unoptimized, and any new
probes registered after that will not be optimized. Note that this
knob *changes* the optimized state. This means that optimized probes
(marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
removed). If the knob is turned on, they will be optimized again.