Re: [RFC] perf record: missing buildid for callstack modules

[Arnaldo Carvalho de Melo]

Em Fri, Jan 15, 2016 at 10:58:48AM -0800, Stephane Eranian escreveu:
> On Fri, Jan 15, 2016 at 1:34 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Thu, Jan 14, 2016 at 05:59:48PM -0800, Stephane Eranian wrote:
> >> Peter,
> >>
> >> On Thu, Jan 14, 2016 at 3:36 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >> > On Thu, Jan 14, 2016 at 12:27:34PM +0100, Ingo Molnar wrote:
> >> >> > +    *      u32                             filename_len;
> >> >> > +    *      char                            filename[2+];
> >> >
> >> >> Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>
> >> >
> >> > except of course that sizeof(u32) == 4 :/
> >
> >> There is no padding here. Are you concerned about running out of bits
> >> in filename_len?
> >
> > No, I just made a mess of it :-)
> >
> > filename_len should have been u16 and filename should then be 6+8n in
> > size.
> >
> why don't you make it more explicit:
>    u16 filename_len
>    u16 extra_len
 
> then it would be clear what is what, no field with dual meaning.
> Now, that assumes that no pathname can be longer than 65535 bytes.

There is no requirement for the filename to start at a u64 boundary,
right? So it can start right after the filename_len, be it u16 or u32,
and that extra_len can be computed as Peter described right here:
 
> >> Any extension possible because header.size - sizeof(mmap3) -
> >> filename_len sizing what's after filename, right?

So, no need to have that extra_len :-)

Ah PATH_MAX is 4096, so I guess u16 should be enough, no?

- Arnaldo

> > Right, current MMAP records use the remaining size as the filename
> > length, but by explicitly specifying that we can add optional fields.
> >
> > These fields must be after filename_len, otherwise you'd not be able to
> > find filename_len and you could not compute the extra size. And given
> > alignment constraints it makes sense to do it after filename[].
> >
> > So suppose we wanted to also add atime and ctime, we could do.
> >
> >   PERF_RECORD_MMAP3 {
> >     ...
> >     u16 filename_len;
> >     char filename[6+8n];
> >
> >     if (extra_size >= 16) {
> >       u64 stime;
> >       u64 ctime;
> >     };
> >   }
> >
> > or something like that.