Messages in this thread | | | Date | Wed, 09 May 2012 18:02:43 +0900 | From | Masami Hiramatsu <> | Subject | Re: [PATCH 6/9][RFC] kprobes: Allow probe on ftrace reserved text (but move it) |
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(2012/05/09 17:12), Masami Hiramatsu wrote: > (2012/05/09 14:53), Masami Hiramatsu wrote: >> (2012/05/08 22:04), Steven Rostedt wrote: >>> Note, the big difference with using ftrace optimization and normal >>> kprobe jump optimization is that the ftrace one can be used on a preempt >>> kernel. But this code is still under development. I want to get a >>> solution for the current code (this patch set) now. It would be nice if >>> it was ready for 3.5. >> >> I doubt that we can really do this. If this is possible, I can make >> jump optimization work with preemptive kernel. > > Now I'm clear that it can be done only with ftrace-nop. > What I concerned with is out-of-line execution buffer releasing > timing. but with the ftrace-nop, we don't need to do that. > > Theoretically, that will be done in following steps; > > 1. call mcount (w/ push call-site address+5) > 2. mcount calls ftrace handlers > 3. mcount makes pt_regs on the stack > - on x86-32, regs->cs should be the call-site address+5, since > regs->sp must be the top of original stack.
Oops, no, %flags is stored on regs->cs and regs->flags is call-site address. So we have to fix it up in optprobe handler.
> - on x86-64, it is enough that regs->sp points the top of original > stack. > 4. mcount store call-site address+5 to %ax > 5. mcount calls new-optprobe handler (which returns call-site address+5) > 6. mcount restores registers > 7. mcount returns to the call-site address+5. > > Between 6 and 7, original opt-probe has to do out-of-line execution, > but this ftrace-based one doesn't need to do that. In that case, > we don't need to allocate nor to release out-of-line execution buffer > for this probe.
And I got an concrete idea of kprobes call-optimization which uses call instead of jump. That will make ftrace-based optimization very easy. :)
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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