Re: [RFC][PATCH -tip 0/9] tracing: kprobe-based event tracer

[Masami Hiramatsu]

Steven Rostedt wrote:
> On Thu, 19 Mar 2009, Masami Hiramatsu wrote:
> 
>> Hi,
>>
>> This is a series of patches which introduce a proof-of concept of
>> kprobe-based event tracer to ftrace. I think that we could port some
>> tracing features from systemtap on this vehicle.
>> This can be applied on the linux-2.6-tip tree.
>>
>> This patchset includes following changes:
>> - Add kprobe-tracer plugin
>> - Add kernel_trap_sp() on x86, ia64, power, s390, arm which are
>>   ported from systemtap runtime.
>> - Add module_*probe api for repawning/removing kprobes when target
>>   module is coming/going.
>>
>> It's still not unclear that the last module_*probe would better be
>> provided as APIs or just embed it in trace_kprobe.c.
>>
>> Future items:
>> - Use binary print.
>> - Add kernel_trap_sp() on other archs.
>> - Support symbol-based memory fetching (for global variables)
>> - Support primitive types(long, ulong, int, uint, etc) for args.
>> - Support indirect memory fetch from register etc.
>> - Check insertion point safety by using instruction decoder.
>>
>> kprobe-based event tracer
>> ---------------------------
>>
>> This tracer is similar to the events tracer which is based on Tracepoint
>> infrastructure. Instead of Tracepoint, this tracer is based on kprobes(kprobe
>> and kretprobe). It probes anywhere where kprobes can probe(this means, all
>> functions body except for __kprobes functions).
>>
>> Unlike the function tracer, this tracer can probe instructions inside of
>> kernel functions. It allows you to check which instruction has been executed.
>>
>> Unlike the Tracepoint based events tracer, this tracer can add new probe points
>> on the fly.
>>
>> Similar to the events tracer, this tracer doesn't need to be activated via
>> current_tracer, instead of that, just set probe points via
>> /debug/tracing/kprobe_probes.
>>
>> Synopsis of kprobe_probes:
>>   p SYMBOL[+offs|-offs]|MEMADDR [FETCHARGS]     : set a probe
>>   r SYMBOL[+0] [FETCHARGS]                      : set a return probe
>>
>>  FETCHARGS:
>>   rN    : Fetch Nth register (N >= 0)
>>   sN    : Fetch Nth entry of stack (N >= 0)
>>   mADDR : Fetch memory at ADDR (ADDR should be in kernel)
>>   aN    : Fetch function argument. (N >= 1)(*)
>>   rv    : Fetch return value.(**)
>>   rp    : Fetch return address.(**)
>>
>>   (*) aN may not correct on asmlinkaged functions and at function body.
>>   (**) only for return probe.
>>
>> E.g.
>>   echo p do_sys_open a1 a2 a3 a4 > /debug/tracing/kprobe_probes
>>
>>  This sets a kprobe on the top of do_sys_open() function with recording
>> 1st to 3rd arguments.
> 
> Do you mean 1st to 4th?

Oops, yes... it records 1st to 4th arguments.

>>   echo r do_sys_open rv rp >> /debug/tracing/kprobe_probes
>>
>>  This sets a kretprobe on the return point of do_sys_open() function with
>> recording return value and return address.
>>
>>   echo > /debug/tracing/kprobe_probes
>>
>>  This clears all probe points. and you can see the traced information via
>> /debug/tracing/trace.
>>
>>   echo /debug/tracing/trace
>> # tracer: nop
>> #
>> #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
>> #              | |       |          |         |
>>            <...>-2376  [001]   262.389131: do_sys_open: @do_sys_open+0 0xffffff9c 0x98db83e 0x8880 0x0
>>            <...>-2376  [001]   262.391166: sys_open: <-do_sys_open+0 0x5 0xc06e8ebb
>>            <...>-2376  [001]   264.384876: do_sys_open: @do_sys_open+0 0xffffff9c 0x98db83e 0x8880 0x0
>>            <...>-2376  [001]   264.386880: sys_open: <-do_sys_open+0 0x5 0xc06e8ebb
>>            <...>-2084  [001]   265.380330: do_sys_open: @do_sys_open+0 0xffffff9c 0x804be3e 0x0 0x1b6
>>            <...>-2084  [001]   265.380399: sys_open: <-do_sys_open+0 0x3 0xc06e8ebb
>>
>>  @SYMBOL means that kernel hits a probe, and <-SYMBOL means kernel returns
>> from SYMBOL(e.g. "sysenter_do_call: <-sys_open+0" means kernel returns from
>> sys_open to sysenter_do_call).
>>
> 
> 
> This looks cool. I'll have to start playing with it.

Thanks!

> 
> Thanks,
> 
> -- Steve
> 

-- 
Masami Hiramatsu

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

e-mail: mhiramat@xxxxxxxxxx

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