lkml.org 
[lkml]   [2008]   [Aug]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 0/5] ftrace: to kill a daemon
* Steven Rostedt (rostedt@goodmis.org) 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
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


\
 
 \ /
  Last update: 2008-08-08 19:25    [W:0.120 / U:0.112 seconds]
©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site