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

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Date:	Thu, 16 Apr 2009 22:13:32 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Frederic Weisbecker <fweisbec@...il.com>
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

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.

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