Re: [dm-devel] [PATCH v7 06/16] tracepoint: use new hashtable implementation

[Josh Triplett <josh joshtriplett org>]

On Mon, Oct 29, 2012 at 01:29:24PM -0400, Sasha Levin wrote:
> On Mon, Oct 29, 2012 at 7:35 AM, Mathieu Desnoyers
> <mathieu desnoyers efficios com> wrote:
> > * Sasha Levin (levinsasha928 gmail com) wrote:
> >> Switch tracepoints to use the new hashtable implementation. This reduces the amount of
> >> generic unrelated code in the tracepoints.
> >>
> >> Signed-off-by: Sasha Levin <levinsasha928 gmail com>
> >> ---
> >>  kernel/tracepoint.c | 27 +++++++++++----------------
> >>  1 file changed, 11 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> >> index d96ba22..854df92 100644
> >> --- a/kernel/tracepoint.c
> >> +++ b/kernel/tracepoint.c
> >> @@ -26,6 +26,7 @@
> >>  #include <linux/slab.h>
> >>  #include <linux/sched.h>
> >>  #include <linux/static_key.h>
> >> +#include <linux/hashtable.h>
> >>
> >>  extern struct tracepoint * const __start___tracepoints_ptrs[];
> >>  extern struct tracepoint * const __stop___tracepoints_ptrs[];
> >> @@ -49,8 +50,7 @@ static LIST_HEAD(tracepoint_module_list);
> >>   * Protected by tracepoints_mutex.
> >>   */
> >>  #define TRACEPOINT_HASH_BITS 6
> >> -#define TRACEPOINT_TABLE_SIZE (1 << TRACEPOINT_HASH_BITS)
> >> -static struct hlist_head tracepoint_table[TRACEPOINT_TABLE_SIZE];
> >> +static DEFINE_HASHTABLE(tracepoint_table, TRACEPOINT_HASH_BITS);
> >>
> > [...]
> >>
> >> @@ -722,6 +715,8 @@ struct notifier_block tracepoint_module_nb = {
> >>
> >>  static int init_tracepoints(void)
> >>  {
> >> +     hash_init(tracepoint_table);
> >> +
> >>       return register_module_notifier(&tracepoint_module_nb);
> >>  }
> >>  __initcall(init_tracepoints);
> >
> > So we have a hash table defined in .bss (therefore entirely initialized
> > to NULL), and you add a call to "hash_init", which iterates on the whole
> > array and initialize it to NULL (again) ?
> >
> > This extra initialization is redundant. I think it should be removed
> > from here, and hashtable.h should document that hash_init() don't need
> > to be called on zeroed memory (which includes static/global variables,
> > kzalloc'd memory, etc).
> 
> This was discussed in the previous series, the conclusion was to call
> hash_init() either way to keep the encapsulation and consistency.
> 
> It's cheap enough and happens only once, so why not?

Unnecessary work adds up.  Better not to do it unnecessarily, even if by
itself it doesn't cost that much.

It doesn't seem that difficult for future fields to have 0 as their
initialized state.

- Josh Triplett