linux-kernel - Re: event tracing, ringbuffer and RB_MAX_SMALL

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Message-ID: <4A5E78EA.60903@cn.fujitsu.com>
Date:	Thu, 16 Jul 2009 08:48:42 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Johannes Berg <johannes@...solutions.net>
CC:	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: event tracing, ringbuffer and RB_MAX_SMALL_DATA

Johannes Berg wrote:
> On Wed, 2009-07-15 at 09:08 +0800, Li Zefan wrote:
> 
>> Right, the length of a dynamic array is not recorded, and this
>> causes 2 problems:
>>
>> - the event filter is not working properly for dynamic strings
>> - userspace parsers can't figure out the length of those arrays
>>
>> I had an idea some time ago, and hopefully will send out a
>> patch today or tomorrow. 
> 
> Well except for the corner case I pointed out, you can determine the
> length of dynamic arrays by either
>  - the next dynamic array's offset or
>  - the length of the item.
> 
> So, afaict, the simplest solution would be to not embed the length of
> the item in type_len if it's not divisible by four and contains dynamic
> members, though the latter condition might be hard to check.
> 

Actually I'm going to encode the size of a dynamic array
in it's @offset, so the lower 16bits is offset and the
higher 16bits is size.

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