Re: [PATCH v2 6/6] kernel: tracepoints: add support for relative references

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 18 Aug 2017 12:44:17 +0100
Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:

> On 18 August 2017 at 12:26, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> > To avoid the need for relocating absolute references to tracepoint
> > structures at boot time when running relocatable kernels (which may
> > take a disproportionate amount of space), add the option to emit
> > these tables as relative references instead.
> >
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> > ---
> >  include/linux/tracepoint.h | 42 ++++++++++++++++++--
> >  kernel/tracepoint.c        |  7 +---
> >  2 files changed, 40 insertions(+), 9 deletions(-)
> >
> > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > index a26ffbe09e71..68701821933a 100644
> > --- a/include/linux/tracepoint.h
> > +++ b/include/linux/tracepoint.h
> > @@ -228,6 +228,42 @@ extern void syscall_unregfunc(void);
> >                 return static_key_false(&__tracepoint_##name.key);      \
> >         }
> >
> > +#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
> > +#define __TRACEPOINT_ENTRY(name)                                       \
> > +       asm("   .section \"__tracepoints_ptrs\", \"a\"\n"               \
> > +           "   .balign 4\n"                                            \
> > +           "   .long " VMLINUX_SYMBOL_STR(__tracepoint_##name) " - .\n"\
> > +           "   .previous\n")
> > +
> > +struct tracepoint_entry_t {
> > +       int tp_offset;
> > +};
> > +
> > +static inline
> > +struct tracepoint *tracepoint_from_entry(const struct tracepoint_entry_t *ent)
> > +{
> > +       return (struct tracepoint *)((unsigned long)ent + ent->tp_offset);
> > +}
> > +#else
> > +#define __TRACEPOINT_ENTRY(name)                                        \
> > +       static struct tracepoint * const __tracepoint_ptr_##name __used  \
> > +       __attribute__((section("__tracepoints_ptrs"))) =                 \
> > +               &__tracepoint_##name
> > +
> > +struct tracepoint_entry_t {
> > +       struct tracepoint *tp;
> > +};
> > +
> > +static inline
> > +struct tracepoint *tracepoint_from_entry(const struct tracepoint_entry_t *ent)
> > +{
> > +       return ent->tp;
> > +}
> > +#endif
> > +
> > +extern struct tracepoint_entry_t const __start___tracepoints_ptrs[];
> > +extern struct tracepoint_entry_t const __stop___tracepoints_ptrs[];
> > +  
> 
> It appears the stuff above needs to be move inside the double-include
> guard (which oddly enough does not cover the entire file)

Why was this moved to the header file? To fulfill some checkpatch
warning?

-- Steve

> 
> >  /*
> >   * We have no guarantee that gcc and the linker won't up-align the tracepoint
> >   * structures, so we create an array of pointers that will be used for iteration
> > @@ -237,11 +273,9 @@ extern void syscall_unregfunc(void);
> >         static const char __tpstrtab_##name[]                            \
> >         __attribute__((section("__tracepoints_strings"))) = #name;       \
> >         struct tracepoint __tracepoint_##name                            \
> > -       __attribute__((section("__tracepoints"))) =                      \
> > +       __attribute__((section("__tracepoints"))) __used =               \
> >                 { __tpstrtab_##name, STATIC_KEY_INIT_FALSE, reg, unreg, NULL };\
> > -       static struct tracepoint * const __tracepoint_ptr_##name __used  \
> > -       __attribute__((section("__tracepoints_ptrs"))) =                 \
> > -               &__tracepoint_##name;
> > +       __TRACEPOINT_ENTRY(name);
> >
> >  #define DEFINE_TRACE(name)                                             \
> >         DEFINE_TRACE_FN(name, NULL, NULL);
> > diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> > index 685c50ae6300..21bc41454fd6 100644
> > --- a/kernel/tracepoint.c
> > +++ b/kernel/tracepoint.c
> > @@ -28,9 +28,6 @@
> >  #include <linux/sched/task.h>
> >  #include <linux/static_key.h>
> >
> > -extern struct tracepoint * const __start___tracepoints_ptrs[];
> > -extern struct tracepoint * const __stop___tracepoints_ptrs[];
> > -
> >  /* Set to 1 to enable tracepoint debug output */
> >  static const int tracepoint_debug;
> >
> > @@ -508,12 +505,12 @@ static void for_each_tracepoint_range(struct tracepoint * const *begin,
> >                 void (*fct)(struct tracepoint *tp, void *priv),
> >                 void *priv)
> >  {
> > -       struct tracepoint * const *iter;
> > +       struct tracepoint_entry_t const *iter;
> >
> >         if (!begin)
> >                 return;
> >         for (iter = begin; iter < end; iter++)
> > -               fct(*iter, priv);
> > +               fct(tracepoint_from_entry(iter), priv);
> >  }
> >
> >  /**
> > --
> > 2.11.0
> >