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

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Date:	Thu, 7 Aug 2008 16:42:16 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Roland McGrath <roland@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Clark Williams <williams@...hat.com>
Subject: Re: [PATCH 0/5] ftrace: to kill a daemon


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.

-- Steve

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