Re: [PATCH] riscv: kprobe: Optimize kprobe with accurate atomicity

[Guo Ren]

On Thu, Feb 16, 2023 at 11:23 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> Hi,
>
> Sorry I missed this thread.
>
> On Tue, 31 Jan 2023 10:33:05 +0000
> Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> > On Tue, Jan 31, 2023 at 09:48:29AM +0800, Guo Ren wrote:
> > > On Mon, Jan 30, 2023 at 11:49 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > > >
> > > > Hi Bjorn,
> > > >
> > > > On Mon, Jan 30, 2023 at 04:28:15PM +0100, Björn Töpel wrote:
> > > > > Guo Ren <guoren@xxxxxxxxxx> writes:
> > > > >
> > > > > >> In the serie of RISCV OPTPROBES [1], it patches a long-jump instructions pair
> > > > > >> AUIPC/JALR in kernel text, so in order to ensure other CPUs does not execute
> > > > > >> in the instructions that will be modified, it is still need to stop other CPUs
> > > > > >> via patch_text API, or you have any better solution to achieve the purpose?
> > > > > >  - The stop_machine is an expensive way all architectures should
> > > > > > avoid, and you could keep that in your OPTPROBES implementation files
> > > > > > with static functions.
> > > > > >  - The stop_machine couldn't work with PREEMPTION, so your
> > > > > > implementation needs to work with !PREEMPTION.
> > > > >
> > > > > ...and stop_machine() with !PREEMPTION is broken as well, when you're
> > > > > replacing multiple instructions (see Mark's post at [1]). The
> > > > > stop_machine() dance might work when you're replacing *one* instruction,
> > > > > not multiple as in the RISC-V case. I'll expand on this in a comment in
> > > > > the OPTPROBES v6 series.
> > > >
> > > > Just to clarify, my comments in [1] were assuming that stop_machine() was not
> > > > used, in which case there is a problem with or without PREEMPTION.
> > > >
> > > > I believe that when using stop_machine(), the !PREEMPTION case is fine, since
> > > > stop_machine() schedules work rather than running work in IRQ context on the
> > > > back of an IPI, so no CPUs should be mid-sequnce during the patching, and it's
> > > > not possible for there to be threads which are preempted mid-sequence.
> > > >
> > > > That all said, IIUC optprobes is going to disappear once fprobe is ready
> > > > everywhere, so that might be moot.
> > > The optprobes could be in the middle of a function, but fprobe must be
> > > the entry of a function, right?
> > >
> > > Does your fprobe here mean: ?
> > >
> > > The Linux kernel configuration item CONFIG_FPROBE:
> > >
> > > prompt: Kernel Function Probe (fprobe)
> > > type: bool
> > > depends on: ( CONFIG_FUNCTION_TRACER ) && (
> > > CONFIG_DYNAMIC_FTRACE_WITH_REGS ) && ( CONFIG_HAVE_RETHOOK )
> > > defined in kernel/trace/Kconfig
> >
> > Yes.
> >
> > Masami, Steve, and I had a chat at the tracing summit late last year (which
> > unfortunately, was not recorded), and what we'd like to do is get each
> > architecture to have FPROBE (and FTRACE_WITH_ARGS), at which point OPTPROBE
> > and KRETPROBE become redundant and could be removed.
>
> No, the fprobe will replace the KRETPROBE but not OPTPROBE. The OPTPROBE
> is completely different one. Fprobe is used only for function entry, but
> optprobe is applied to the function body.
>
> >
> > i.e. we'd keep KPROBES as a "you can trace any instruction" feature, but in the
> > few cases where OPTPROBES can make things fater by using FTRACE, you should
> > just use that directly via FPROBE.
>
> I think what you are saying is KPROBE_ON_FTRACE, and that will be replaced by
> FPROBES.
I'm sorry, I'm not sure how FPROBES could replace KPROBE_ON_FTRACE? Do
you have some discussion on it?

>
> Thank you,
>
> >
> > Thanks,
> > Mark.
>
>
> --
> Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>



-- 
Best Regards
 Guo Ren