linux-kernel - Re: [PATCH 0/9] jump label v6

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Date:	Fri, 09 Apr 2010 16:36:24 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Jason Baron <jbaron@...hat.com>
CC:	linux-kernel@...r.kernel.org, mingo@...e.hu,
	mathieu.desnoyers@...ymtl.ca, hpa@...or.com, tglx@...utronix.de,
	rostedt@...dmis.org, andi@...stfloor.org, roland@...hat.com,
	rth@...hat.com, fweisbec@...il.com, avi@...hat.com
Subject: Re: [PATCH 0/9] jump label v6

Hi Jason,

Jason Baron wrote:
> Hi,
> 
> Refresh of jump labeling patches aginst -tip tree. For bacground see: 
> http://marc.info/?l=linux-kernel&m=125858436505941&w=2
> 
> I believe I've addressed all the reviews from v5.
> 
> Changes in v6:
> 
> * I've moved Steve Rostedt's 'ftrace_dyn_arch_init()' to alternative.c to
> put it into a common area for used by both ftrace and jump labels. By
> default we put a 'jmp 5' in the nop slot. Then, when we detect the best
> runtime no-op we patch over the 'jmp 5' with the appropriate nop.
> 
> * build time sort of the jump label table. The jump label table is more
> optimally accessed if the entries are continguous. Sorting the table
> accomplishes this. Do the sort at build-time. Adds a '-j' option to 
> 'modpost' which replaces the vmlinux, with a sorted jump label section vmlinux.
> I've tested this on x86 with relocatable and it works fine there as well. Note
> that I have not sorted the jump label table in modules. This is b/c the jump
> label names can be exported by the core kernel, and thus I don't have them
> available at buildtime. This could be solved by either finding the correct
> ones in the vmlinux, or by embedding the name of the jump label in the module
> tables (and not just a pointer), but the module tables tend to be smaller, and
> thus there is less value to this kind of change anyway. The kernel continues to
> do the sort, just in case, but at least for the vmlinux, this is just a
> verfication b/c the jump label table has already been sorted.
> 
> * added jump_label_text_reserved(), so that other routines that want to patch
> the code, can first verify that they are not stomping on jump label addresses.

Good!:-)
So now, it might be a good time to integrate those text_reserved() functions.

BTW, how many jumps would you expect modifying at once?
Since the text_poke_smp() uses stop_machine() for each modifying text,
I found that it can cause a delay issue if it is called so many times...
(e.g. a systemtap testcase sets ~5000 probes at once)

Thank you,

-- 
Masami Hiramatsu
e-mail: mhiramat@...hat.com
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