Messages in this thread | | | Date | Thu, 11 Aug 2011 15:28:44 +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 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.
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).
... are there any other cons? :)
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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