Re: [PATCH 0/5] ftrace: to kill a daemon

[Mathieu Desnoyers]

* Steven Rostedt (rostedt@xxxxxxxxxxx) wrote:
> 
> On Thu, 7 Aug 2008, Mathieu Desnoyers wrote:
> 
> > 
> > Hi Steven,
> > 
> > If you really want to stop using stop_machine, I think you should have a
> > look at my immediate values infrastructure :
> > 
> > see:
> > http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=blob;f=arch/x86/kernel/immediate.c;h=0958c02b49eed3bbc00bdc5aceeee17a6d0c7ab4;hb=HEAD
> > 
> > particularly replace_instruction_safe(), which uses a temporary
> > breakpoint to safely replace an instruction on a live SMP system without
> > using stop_machine. Feel free to try it in ftrace. :)
> > 
> > You may need to tweak the imv_notifier(), which is responsible for
> > executing the breakpoint. The only special thing this handler has to
> > know is the non-relocatable instructions you might want to insert (it
> > only cares about the new instructions, not the old ones being replaced).
> > The current code deals with 5-bytes jumps. Note that the instruction is
> > executed in the breakpoint handler with preemption disabled, which might
> > not be well suited for a call instruction.
> > 
> > I would recommend to patch in a 5-bytes jmp with 0 offset
> > (e9 00 00 00 00) when disabled (this makes a single 5-bytes instruction
> > and thus makes sure no instruction pointer can iret in the middle).
> > 
> > When enabling the site, you could patch-in the original call, but you
> > should tweak the imv_notifier() so that it uses the jmp offset 0 in the
> > bypass instead of the function call, because preemption would otherwise
> > be disabled around the call when executed in the breakpoint bypass.
> > 
> > Therefore, simply statically forcing the bypass code to e9 00 00 00 00
> > in all the cases (a nop) would do the job for your needs.
> 
> I originally used jumps instead of nops, but unfortunately, they actually 
> hurt performance more than adding nops. Ingo told me it was probably due
> to using up the jump predictions of the CPU.
> 

Hrm, are you sure you use a single 5-bytes nop instruction then, or do
you use a mix of various nop sizes (add_nops) on some architectures ?

You can consume the branch prediction buffers for conditional branches,
but I doubt static jumps have this impact ? I don't see what "jump
predictions" you are referring to here exactly.

Mathieu

> -- Steve
> 

-- 
Mathieu Desnoyers
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