Messages in this thread | | | Date | Fri, 12 Aug 2011 11:57:00 +0900 | From | Masami Hiramatsu <> | Subject | Re: [PATCH 0/5][RFC] kprobes/ftrace: Have kprobes use ftrace on ftrace nops |
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(2011/08/11 22:01), Steven Rostedt wrote: > On Thu, 2011-08-11 at 15:28 +0900, Masami Hiramatsu wrote: >> (2011/08/11 9:34), Steven Rostedt wrote: >>> On Thu, 2011-08-11 at 09:21 +0900, Masami Hiramatsu wrote: >>>> Hi Steven, >>>> >>>> Thanks for this nice feature! >>>> >>>> (2011/08/11 1:22), Steven Rostedt wrote: >>>>> Hi All, >>>>> >>>>> I started working on adding the -mfentry switch to ftrace, which >>>>> allows us to remove the frame pointers requirement from function tracing >>>>> as well as makes mcount (fentry) work just better. >>>>> >>>>> But when I did this in another branch, I noticed that I broke kprobes >>>>> in its most common usage. The attaching a probe at the beginning of >>>>> a function to use get to its parameters. >>>>> >>>>> So I started this branch. This branch is to have kprobes use ftrace >>>>> directly when a probe is attached to a ftrace nop. Currently, kprobes >>>>> will just error when that happens. With this patch set, it will hook >>>>> into the ftrace infrastructure and use ftrace instead. This is more >>>>> like an optimized probe as no breakpoints need to be set. A call to >>>>> the function is done directly via the mcount trampoline. If ftrace >>>>> pt_regs is implemented for an arch, kprobes gets this feature for free. >>>> >>>> I agreed this idea, this looks good to me too :) >>>> With -fentry, this can improve dynamic trace events very much. >>>> >>>> BTW (OT), it seems that current kprobe data structure becomes a bit >>>> fat. Maybe what we need is just a "holder of hooking handler" as >>>> what ftrace provides, not a full storage data structure of copied >>>> instrucutions. Perhaps, we'd better diet the kprobe structure for >>>> transparency of hooking infrastructure. >>> >>> Sure, I can make the ftrace_ops field in kprobes dynamically allocated >>> instead. That shouldn't be an issue. >> >> By the way (again), perhaps, much simpler solution is using ftrace >> not in kprobe, but in the trace_kprobe. Of course, there are several >> pros and cons... >> >> The pros: >> - Arch independent solution (anyway, ftrace still needs passing pt_regs >> to their handler) >> - Don't need to introduce more complexity into kprobes itself. >> - Maybe systemtap also can catch up with this as using same method. > > Note that systemtap and others will be hooking into kprobes version, not > the trace_kprobe one. If we do it in trace_kprobe, then everyone else > needs to reimplement it too. I have bigger ideas for the future of > this, and I really want to get this working. If it doesn't work for > kprobes, then it won't work for anything else.
I don't think it won't work. It can work but on a long way. Could you tell me your "bigger ideas"? Perhaps, we are on the different way but aim to same goal.
>> The cons: >> - Native kprobes users will be disappointed... anyway, they just need to >> move their probes to the next instruction (usually addr+=5 is OK). > > I've been told that doing the addr+=5 (which is also arch specific) can > break things for other tools.
As I told in previous mail, I think kprobes can do that transparently.
Thank you,
-- Masami HIRAMATSU Software Platform Research Dept. Linux Technology Center Hitachi, Ltd., Yokohama Research Laboratory E-mail: masami.hiramatsu.pt@hitachi.com
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