On Fri, Jun 24, 2016 at 12:04:40PM -0400, Steven Rostedt wrote:
> On Fri, 24 Jun 2016 15:35:44 +0900
> Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
>
> > > > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> > > > index dea12a6e413b..35c523ba5c59 100644
> > > > --- a/include/linux/ftrace.h
> > > > +++ b/include/linux/ftrace.h
> > > > @@ -751,25 +751,33 @@ extern void ftrace_init(void);
> > > > static inline void ftrace_init(void) { }
> > > > #endif
> > > >
> > > > +#ifndef CONFIG_HAVE_64BIT_ALIGNED_ACCESS
> > > > +# define FTRACE_ALIGNMENT 4
> > > > +#else
> > > > +# define FTRACE_ALIGNMENT 8
> > > > +#endif
> > >
> > > Swap the above. Having the #ifndef is more confusing to understand than
> > > to have a #ifdef.
> >
> > Will do.
> >
> > >
> > > > +
> > > > +#define FTRACE_ALIGN_DATA __attribute__((packed, aligned(FTRACE_ALIGNMENT)))
> > >
> > > Do we really need to pack it? I mean, just get rid of the hole (like
> > > you did with the movement of the overrun) and shouldn't the array be
> > > aligned normally without holes, if the arch can support it? Doesn't gcc
> > > take care of that?
> >
> > I'm not sure I understood you correctly. AFAIK the size of struct is
> > a multiple of alignment unit and gcc manual says the aligment
> > attribute only can be increased unless the 'packed' is used as well..
>
> Ah, I see you are trying to get the recorded size in the array down to
> a 4 byte alignment (due to the "int depth"), instead of adding the 4
> bytes to the buffer.
>
> Hmm, I wondering if we need the ifdef above, as the ring buffer itself
> will force the 8 byte alignment of structures added to the buffer.
As far as I can see, the ring buffer has following code in ring_buffer.c:
#define RB_ALIGNMENT 4U
#define RB_MAX_SMALL_DATA (RB_ALIGNMENT * RINGBUF_TYPE_DATA_TYPE_LEN_MAX)
#define RB_EVNT_MIN_SIZE 8U /* two 32bit words */
#ifndef CONFIG_HAVE_64BIT_ALIGNED_ACCESS
# define RB_FORCE_8BYTE_ALIGNMENT 0
# define RB_ARCH_ALIGNMENT RB_ALIGNMENT
#else
# define RB_FORCE_8BYTE_ALIGNMENT 1
# define RB_ARCH_ALIGNMENT 8U
#endif
#define RB_ALIGN_DATA __aligned(RB_ARCH_ALIGNMENT)
Thanks,
Namhyung
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