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SubjectRe: [PATCH 1/2 v2] tracing/events: provide string with undefined size support
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Le 17 avril 2009 11:29, Peter Zijlstra <a.p.zijlstra@chello.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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