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

[Frédéric Weisbecker]

Le 17 avril 2009 12:04, Frédéric Weisbecker <fweisbec@xxxxxxxxx> a écrit :
> Le 17 avril 2009 11:29, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> 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
>

I messed up it below, but I guess you understood it.
Well just some fixes to be sure everyone understood the idea:

entry = get_entry_from_buffer();
offset = sizeof(struct plop);
entry->bar = entry + offset;
offset += strlen(str1) + 1;
entry->bar2 = entry + offset;
strcpy(entry->bar, str1);
strcpy(entry->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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