linux-kernel - Re: [PATCH 1/2 v2] tracing/events: provide string with undefined size support

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Date:	Thu, 16 Apr 2009 22:28:46 +0200
From:	Frédéric Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	Zhaolei <zhaolei@...fujitsu.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Li Zefan <lizf@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2 v2] tracing/events: provide string with undefined size 
	support

2009/4/16 Peter Zijlstra <a.p.zijlstra@...llo.nl>:
> On Thu, 2009-04-16 at 22:00 +0200, Frederic Weisbecker wrote:
>> Impact: less memory usage for tracing
>>
>> This patch provides the support for dynamic size strings on
>> event tracing.
>>
>> The key concept is to use a structure with an ending char array field of
>> undefined size and use such ability to allocate the minimal size on the ring
>> buffer to make the entry fit inside as opposite to a fixed length strings with
>> upper bound.
>>
>> This patch provides one new macro:
>>
>> -__ending_string(name, src)
>>
>> This one declares the string to the structure inside TP_STRUCT__entry.
>> You need to provide the name of the string field and the source that will be
>> copied inside.
>> Two constraints: only one __ending_string() per TRACE_EVENT can be added and
>> it must be the last field to be declared. Hence the __ending prefix.
>>
>> This macro will declare the necessary field and will also add the dynamic
>> size of the string needed for the ring buffer entry allocation.
>>
>> It also support filtering because these strings behave essentially
>> like usual fixed length string.
>
> can't we simply do __string(name, src) and output something like:
>
>  struct {
>   u16 size;
>   char str[0];
>  } name;
>
> That would get rid of this __ending_ wart.
>
>

Hmm, I don't understand.
Such a thing doesn't seem to work. Once we fill the string it would
override the fields that
follow it if it's not at the end.
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