linux-kernel - Re: [RFC PATCH v4] Unified trace buffer

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Message-ID: <48DD1475.1090109@redhat.com>
Date:	Fri, 26 Sep 2008 12:57:25 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	prasad@...ux.vnet.ibm.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Martin Bligh <mbligh@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH v4] Unified trace buffer

Peter Zijlstra wrote:
> On Fri, 2008-09-26 at 06:45 -0400, Steven Rostedt wrote:
>> On Fri, 26 Sep 2008, Peter Zijlstra wrote:
>>> On Thu, 2008-09-25 at 23:20 -0400, Mathieu Desnoyers wrote:
>>>> You could also fallback on a 2-level page array when buffer size is >
>>>> 64MB. The cost is mainly a supplementary pointer dereference, but one
>>>> more should not make sure a big difference overall.
>>> I'm still not sure why we don't just link the pages using the page
>>> frames, we don't need the random access, do we?
>> Yeah we can go back to that (as ftrace does).
>>
>> 1) It can be very error prone. I will need to encapsulate the logic more.
> 
> Sure.
> 
>> 2) I'm still not sure if crash can handle it.
> 
> It ought to, and if it can't it should be fixed. Having easy access to
> the pageframes is vital to debugging VM issues. So I'd not bother about
> this issue too much.
> 
>> I was going to reply to Masami with this answer, but it makes things more 
>> complex.  For v1 (non RFC v1) I wanted to start simple. v2 can have this 
>> enhancement.
> 
> Right - I just object to having anything vmalloc.

I just requested that the expansion of buffer size limitation too. :)

I don't stick with vmalloc. If that (page frame chain?) can
achieve better performance, I agree that trace buffer uses it.

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

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

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