linux-kernel - Re: Re: [PATCH -tip v9 25/26] kprobes: Introduce kprobe cache to reduce cache misshits

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Date:	Fri, 25 Apr 2014 18:43:29 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>,
	Frederic Weisbecker <fweisbec@...il.com>, x86@...nel.org,
	Steven Rostedt <rostedt@...dmis.org>, fche@...hat.com,
	mingo@...hat.com, systemtap@...rceware.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Re: [PATCH -tip v9 25/26] kprobes: Introduce kprobe cache
 to reduce cache misshits

(2014/04/25 17:20), Ingo Molnar wrote:
> 
> * Masami Hiramatsu <masami.hiramatsu.pt@...achi.com> wrote:
> 
>>> So I don't think this should be a Kconfig entry, just enable it 
>>> unconditionally. That will further simplify the code.
>>
>> Hmm, it consumes some amount of memory (36KB/core) just for the case 
>> of several thousand of kprobes. On enterprise servers and desktop 
>> it's OK, no problem. But I think, some embedded systems with small 
>> resources will not want that. [...]
> 
> They'll just disable kprobes in general.

No, I'd like to provide kprobes (and dynamic events) to them (including
me) for debugging and dynamic monitoring, instead of modifying code for
adding events on their kernel. To solve some specific issues, specific
events (not generic events) are required. Making local patches to add
such events is an option, but it increases maintenance cost for rebasing.
It is better to pay cost to maintain this kconfig on upstream as the
maintainer for me instead of paying such ugly local cost. :(

Anyway, this option is not easy for beginners, I think it should be
defined with "if EXPERT" option and make it enabled by default.

> Really, at this point complexity is our main concern.

Agreed about complexity issue. However, even if we remove the Kconfig,
we can just save 6 lines of the code, and one #ifdef block.
Can that really solve the complexity problem?

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com

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