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

[David Howells <dhowells@redhat.com>]

Ingo Molnar <mingo@elte.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