linux-kernel - Re: [RFC] timers, pointers to functions and type safety

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Message-ID: <28665.1165234964@redhat.com>
Date:	Mon, 04 Dec 2006 12:22:44 +0000
From:	David Howells <dhowells@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Al Viro <viro@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Matthew Wilcox <matthew@....cx>,
	Linus Torvalds <torvalds@...l.org>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] timers, pointers to functions and type safety 

Ingo Molnar <mingo@...e.hu> wrote:

> the question is: which is more important, the type safety of a 
> container_of() [or type cast], which if we get it wrong produces a 
> /very/ trivial crash that is trivial to fix - or embedded timers data 
> structure size all around the kernel? I believe the latter is more 
> important.

Indeed yes.

Using container_of() and ditching the data value, you generally have to have
one extra instruction per timer handler, if that, but you are able to discard
one instruction or more from __run_timers() and struct timer_list discards a
word.

You will almost certainly have far more timer_list structs in the kernel than
timer handler functions, therefore it's a space win, and possibly also a time
win (if the reduction of __run_timers() is greater than the increase in the
timer handler).

And that extra instruction in the timer handler is usually going to be an
addition or subtraction of a small immediate value - which may be zero (in
which case the insn is dropped) or which may be folded directly into memory
access instruction offsets.

David
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