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

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Date:	Fri, 17 Apr 2009 12:04:00 +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

Le 17 avril 2009 11:29, Peter Zijlstra <a.p.zijlstra@...llo.nl> a écrit :
> On Fri, 2009-04-17 at 10:59 +0200, Frédéric Weisbecker wrote:
>
>> struct foo {
>>    field1;
>>    field2;
>>    ...
>> };
>>
>> struct foo *f;
>>
>> event = ring_buffer_lock_reserve(sizeof(*f), ...);
>> f = ring_buffer_event_data(event);
>>
>> I can't use a kmalloc here. We are tracing a random event which can happen
>> at a random frequency, random context, etc...
>
> Can't you add a bit to that ring_buffer_lock_reserve() thing?
>
> The thing I do for perf_counters is I first iterate all the output, then
> make the reserve large enough to fit all the output in, then copy the
> bits into the output buffer.
>
> The result is that the output cannot be interpreted as a fixed offset
> struct, but that's not much of an issue anyway.
>
> Another possibility is using relative pointers for strings that point
> beyond the tail of the fixed offset struct.
>
> So something like:
>
>  __field(int, foo);
>  __string(bar);
>  __field(int, foo2);
>  __string(bar2);
>  __field(int, foo3);
>
> would look like:
>
>  struct plop {
>   int foo;
>   char *bar;
>   int foo2;
>   char *bar2;
>   int foo3;
>
>   char data[0];
>  }
>
> and you'd do something like:
>
>  size = sizeof(struct plop);
>  size += strlen(bar) + 1;
>  size += strlen(bar2) + 1;
>
>  event = ring_buffer_lock_reserve(size);
>  offset = sizeof(struct plop);
>  my_plop.bar = (char *)offset;
>  offset += strlen(bar) + 1;
>  my_plop.bar2 = (char *)offset;
>  memcpy(&event, &my_plop, sizeof(struct plop));
>  memcpy(&event + my_plop.bar, bar, strlen(bar)+1);
>  memcpy(&event + my_plop.bar2, bar2, strlen(bar2)+1);
>  ring_buffer_unlock();
>
> Then on reading, you'd get a variable sized entry, with a fixed size
> fixed offset struct, that contains relative offset character pointers.



I like much this idea.
Just a small change on it: I could use absolute addresses

offset = sizeof(struct plop);
my_plop.bar = &entry + offset;
offset += strlen(str1) + 1;
my_plop.bar2 = &entry + offset;
strcpy(my_plop.bar, str1);
strcpy(my_plop.bar2, str2);

So that its integration will need very few changes to support printing
and filtering. It will just behave like usual char [..].

Nice! Thanks Peter.
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