linux-kernel - Re: [PATCH 1/5] ftrace: Synchronize variable setting with breakpoints

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Date:	Thu, 31 May 2012 21:06:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"H. Peter Anvin" <hpa@...or.com>, Dave Jones <davej@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH 1/5] ftrace: Synchronize variable setting with
 breakpoints

On Thu, 2012-05-31 at 14:50 -0400, Steven Rostedt wrote:

> > Well, the fail is before that, how could we grow two pieces of code
> > doing similar things in the first place? 
> 
> Again, ftrace is slightly different as it does 30,000 changes at once,
> on top of known nops. This was done through stop_machine(), thus any
> slowdown was a large hit to system performance. text_poke() took the way
> of mapping a page to do the change, and Mathieu didn't want to change
> that (IIRC). But now we want the two to be similar.

We could give text_poke a function argument to do the actual
modification, leaving all the magic centralized.

Also, why did Mathieu insist on keeping that kmap()?

> > I hardly ever use dyn-ftrace but I do use some text_poke() through
> > jump_labels.
> 
> You don't use function tracer? That's dyn-ftrace.

Not much no.. I do use trace_printk() and ftrace_dump_on_oops a lot
though.

> But still, we need to keep the record as small as possible because it is
> persistent throughout the life of the system running. Every location
> must be recorded, and maintain a state (flags).
> 
> Text_poke() mostly grew out of the jump-label work. But yes, there's
> still a lot that can be shared. The actual code modification may be.

Afaicr we didn't change text_poke() for the jump-label stuff, except in
trivial ways (added a #ifdef and exposed a function etc..).

> > I would still like to end up with one code base doing CMC with two
> > implementations depending on a Kconfig knob.
> 
> You mean keep stop_machine around?

Yeah, like have CONFIG_CMC_STOPMACHINE and CONFIG_CMC_FANCY for a little
while.

If we find a problem with the fancy approach going back is easy, once
its proven stable we could remove the stop-machine one.
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