linux-kernel - Re: [PATCH v7 06/16] tracepoint: use new hashtable implementation

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Message-ID: <20121029191042.GA21864@Krystal>
Date:	Mon, 29 Oct 2012 15:10:42 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Sasha Levin <levinsasha928@...il.com>,
	Josh Triplett <josh@...htriplett.org>,
	torvalds@...ux-foundation.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,
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Subject: Re: [PATCH v7 06/16] tracepoint: use new hashtable implementation

* Tejun Heo (tj@...nel.org) wrote:
> On Mon, Oct 29, 2012 at 11:58:14AM -0700, Tejun Heo wrote:
> > On Mon, Oct 29, 2012 at 02:53:19PM -0400, Mathieu Desnoyers wrote:
> > > The argument about hash_init being useful to add magic values in the
> > > future only works for the cases where a hash table is declared with
> > > DECLARE_HASHTABLE(). It's completely pointless with DEFINE_HASHTABLE(),
> > > because we could initialize any debugging variables from within
> > > DEFINE_HASHTABLE().
> > 
> > You can do that with [0 .. HASH_SIZE - 1] initializer.
> 
> And in general, let's please try not to do optimizations which are
> pointless.  Just stick to the usual semantics.  You have an abstract
> data structure - invoke the initializer before using it.  Sure,
> optimize it if it shows up somewhere.  And here, if we do the
> initializers properly, it shouldn't cause any more actual overhead -
> ie. DEFINE_HASHTABLE() will basicallly boil down to all zero
> assignments and the compiler will put the whole thing in .bss anyway.

Yes, agreed. I was going too far in optimization land by proposing
assumptions on zeroed memory. All I actually really care about is that
we don't end up calling hash_init() on a statically defined (and thus
already initialized) hash table.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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