linux-kernel - Re: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this is for x86

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Date:	Sat, 19 Feb 2011 14:07:24 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	Dominique Toupin <dominique.toupin@...csson.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"2nddept-manager@....hitachi.co.jp" 
	<2nddept-manager@....hitachi.co.jp>
Subject: Re: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this is
 for x86_64 right now)

(2011/02/19 8:02), Steven Rostedt wrote:
> On Fri, 2011-02-18 at 14:45 -0800, H. Peter Anvin wrote:
> 
>> We should also be able to use the breakpoint hack to avoid holding all
>> the CPUs.  They still need to be interrupted, but that skips the
>> rendezvous operation.

That's what I've done with text_poke_fixup()
http://lkml.org/lkml/2009/12/18/312

And I think it still not be checked officially from silicon side.

> As this is about the ftrace code, I'm in the process of analyzing and
> updating how the function tracer works. I can look to see if I can
> design it so we don't have to always use stop_machine() if a breakpoint
> method is in place.
> 
> Basically what is needed is to convert a "nop" into a "call" or maybe
> the other way around, safely.
> 
> Now is it safe to insert a breakpoint (usually a byte I believe), modify
> the rest of the instruction and then replace the breakpoint to the new
> code? Since the instruction that is being replaced or the instruction
> being added is always a nop, the breakpoint handler needs to do nothing
> but return to the location after the nop/call.

Yes, at least with text_poke_fixup(), you can call it as below

text_poke_fixup(addr, call_insn, CALL_INSN_SIZE, addr + CALL_INSN_SIZE);

Then, if a processor hits the addr, the breakpoint handler changes its
regs->ip to addr + CALL_INSN_SIZE so that it skips the modifying
instruction.

> Is there any synchronization that needs to be done when doing this? Or
> can it just be:
> 
> 	insert_breakpoint();
> 	update_instruction();
> 	remove_breakpoint();
> 
> Because we need to do this for 22,000 calls in a row.

In the case of text_poke_fixup(), it sends IPI twice for synchronization,
which doesn't stop all cores but current core. Of course, theoretically
this can be reduced by doing it in a batch.

Thank you,

-- 
Masami HIRAMATSU
2nd Dept. Linux Technology Center
Hitachi, Ltd., Systems Development Laboratory
E-mail: masami.hiramatsu.pt@...achi.com
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