linux-kernel - Re: [PATCH 1/2 v3] tracing/events: provide string with undefined size support

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Message-ID: <alpine.DEB.2.00.0904211432040.10097@gandalf.stny.rr.com>
Date:	Tue, 21 Apr 2009 14:33:04 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Ingo Molnar <mingo@...e.hu>, 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>,
	Peter Zijlstra <peterz@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/2 v3] tracing/events: provide string with undefined
 size support


On Tue, 21 Apr 2009, Frederic Weisbecker wrote:

> On Sun, Apr 19, 2009 at 07:01:34AM +0200, Frederic Weisbecker wrote:
> > 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 one or more string entries fit inside, as opposite
> > to a fixed length strings with upper bound.
> > 
> > The strings themselves are represented using fields which have an offset
> > value from the beginning of the entry.
> > 
> > This patch provides three new macros:
> > 
> > __string(item, src)
> > 
> > This one declares a 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.
> > This will also add the dynamic size of the string needed for the ring
> > buffer entry allocation.
> > A stack allocated structure is used to temporarily store the offset
> > of each strings, avoiding double calls to strlen() on each event
> > insertion.
> > 
> > __get_str(field)
> > 
> > This one will give you a pointer to the string you have created. This
> > is an abstract helper to resolve the absolute address given the field
> > name which is a relative address from the beginning of the trace_structure.
> > 
> > __assign_str(dst, src)
> > 
> > Use this macro to automatically perform the string copy from src to
> > dst. src must be a variable to assign and dst is the name of a __string
> > field.
> > 
> > Example on how to use it:
> > 
> > TRACE_EVENT(my_event,
> > 	TP_PROTO(char *src1, char *src2),
> > 
> > 	TP_ARGS(src1, src2),
> > 	TP_STRUCT__entry(
> > 		__string(str1, src1)
> > 		__string(str2, src2)
> > 	),
> > 	TP_fast_assign(
> > 		__assign_str(str1, src1);
> > 		__assign_str(str2, src2);
> > 	),
> > 	TP_printk("%s %s", __get_str(src1), __get_str(src2))
> > )
> > 
> > Of course you can mix-up any __field or __array inside this
> > TRACE_EVENT. The position of the __string or __assign_str
> > doesn't matter.
> > 
> > Changes in v2:
> > 
> > Address the suggestion of Steven Rostedt: drop the opening_string() macro
> > and redefine __ending_string() to get the size of the string to be copied
> > instead of overwritting the whole ring buffer allocation.
> > 
> > Changes in v3:
> > 
> > Address other suggestions of Steven Rostedt and Peter Zijlstra with
> > some changes: drop the __ending_string and the need to have only one
> > string field.
> > Use offsets instead of absolute addresses.
> > 
> > [ Impact: better usage of memory for string tracing ]
> > 
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Li Zefan <lizf@...fujitsu.com>
> > Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > ---
> 
> 
> Hi,
> 
> Do you have doubts about this?
> It would be nice to have a (N)Acked-by from you for these
> two patches :-)

Ouch, I totally missed seeing this. It was engulfed in spam/unrelated 
email.

I'm about to go to school for a few hours, I'll review it when I get back.

Thanks,

-- Steve

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