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Date:	Mon, 29 Oct 2012 14:42:33 -0400
From:	Sasha Levin <levinsasha928@...il.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	torvalds@...ux-foundation.org, tj@...nel.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, paul.gortmaker@...driver.com,
	davem@...emloft.net, rostedt@...dmis.org, mingo@...e.hu,
	ebiederm@...ssion.com, aarcange@...hat.com, ericvh@...il.com,
	netdev@...r.kernel.org, eric.dumazet@...il.com, axboe@...nel.dk,
	agk@...hat.com, dm-devel@...hat.com, neilb@...e.de,
	ccaulfie@...hat.com, teigland@...hat.com,
	Trond.Myklebust@...app.com, bfields@...ldses.org,
	fweisbec@...il.com, jesse@...ira.com,
	venkat.x.venkatsubra@...cle.com, ejt@...hat.com,
	snitzer@...hat.com, edumazet@...gle.com, linux-nfs@...r.kernel.org,
	dev@...nvswitch.org, rds-devel@....oracle.com, lw@...fujitsu.com
Subject: Re: [PATCH v7 06/16] tracepoint: use new hashtable implementation

On Mon, Oct 29, 2012 at 2:31 PM, Josh Triplett <josh@...htriplett.org> wrote:
> 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@...icios.com> wrote:
>> > * Sasha Levin (levinsasha928@...il.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@...il.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.

Let's put it this way: hlist requires the user to initialize hlist
head before usage, therefore as a hlist user, hashtable implementation
must do that.

We do it automatically when the hashtable user does
DEFINE_HASHTABLE(), but we can't do that if he does
DECLARE_HASHTABLE(). This means that the hashtable user must call
hash_init() whenever he uses DECLARE_HASHTABLE() to create his
hashtable.

There are two options here, either we specify that hash_init() should
only be called if DECLARE_HASHTABLE() was called, which is confusing,
inconsistent and prone to errors, or we can just say that it should be
called whenever a hashtable is used.

The only way to work around it IMO is to get hlist to not require
initializing before usage, and there are good reasons that that won't
happen.


Thanks,
Sasha
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