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

On Fri, 8 Aug 2008, Mathieu Desnoyers wrote:

> * Steven Rostedt (rostedt@goodmis.org) wrote:
> > > >
> > >
> > > That's bad :
> > >
> > > #define GENERIC_NOP5 GENERIC_NOP1 GENERIC_NOP4
> > >
> > > #define K8_NOP5 K8_NOP3 K8_NOP2
> > >
> > > #define K7_NOP5 K7_NOP4 ASM_NOP1
> > >
> > > So, when you try, later, to replace these instructions with a single
> > > 5-bytes instruction, a preempted thread could iret in the middle of your
> > > 5-bytes insn and cause an illegal instruction ?
> >
> > That's why I use kstop_machine.
> >
>
> kstop_machine does not guarantee that you won't have _any_ thread
> preempted with IP pointing exactly in the middle of your instructions
> _before_ the modification scheduled back in _after_ the modification and
> thus causing an illegal instruction.
>
> Still buggy. :/

Hmm, good point. Unless...

Can a processor be preempted in a middle of nops? What do nops do for a
processor? Can it skip them nicely in one shot?

This means I'll have to do the benchmarks again, and see what the
performance difference of a jmp and a nop is significant. I'm thinking
that if the processor can safely skip nops without any type of processing,
this may be the reason that nops are better than a jmp. A jmp causes the
processor to do a little more work.

I might even run a test to see if I can force a processor that uses the
three-two nops to preempt between them.

I can add a test in x86 ftrace.c to check to see which nop was used, and
use the jmp if the arch does not have a 5 byte nop.

I'm assuming that jmp is more expensive than the nops because otherwise
a jmp 0 would have been used as a 5 byte nop.

-- Steve


\
 
 \ /
  Last update: 2008-08-08 20:43    [W:0.098 / U:0.388 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site