Subject: Re: [patch 00/21] hrtimer - High-resolution timer subsystem
From: Thomas Gleixner <tglx@linutronix.de>
Reply-To: tglx@linutronix.de
To: Roman Zippel <zippel@linux-m68k.org>
Cc: linux-kernel@vger.kernel.org, Andrew Morton <akpm@osdl.org>,
       rostedt@goodmis.org, johnstul@us.ibm.com, mingo@elte.hu
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Organization: linutronix
Date: Mon, 12 Dec 2005 19:37:53 +0100
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> On Mon, 2005-12-12 at 14:39 +0100, Roman Zippel wrote:
> > > Actually the change adds more code lines and removes one field of the
> > > hrtimer structure, but it has exactly the same functionality: Fast
> > > access to the first expiring timer without walking the rb_tree.
> > 
> > Together with the state field this would save 12 bytes, which is IMO very 
> > well worth considering.
> > You seem to have some plans for it, the best hint I've found for it is:
> > 
> > + (This seperate list is also useful for high-resolution timers where we
> > + need seperate pending and expired queues while keeping the time-order
> > + intact.)"
> > 
> > Could you please elaborate on this?
> 
> Sure. I have already removed the list_head for the non high resolution
> case as it turned out that it does not hurt the high resolution
> implementation.
> 
> For the high resolution implementation we have to move the expired
> timers to a seperate list, as we do not want to do complex callback
> functions from the event interrupt itself. But we have to reprogramm the
> next event interrupt, so we need simple access to the timer which
> expires first.
> 
> The initial implementation did simply move the timer from the pending
> list to the expired list without doing the rb_tree removal inside of the
> event interrupt handler. That way the next event for reprogramming was
> the first event in the pending list.
> 
> The new rebased version with the pending list removed does the rb_tree
> removal inside the event interrupt and enqueues the timer, for which the
> callback function has to be executed in the softirq, to the expired
> list. One exception are simple wakeup callback functions, as they are
> reasonably fast and we save two context switches. The next event for
> reprogramming the event interrupt is retrieved by the pointer in the
> base structure.
> 
> This way the list head is only necessary for the high resolution case.
> 
> The state field is not removed

Oops, I somehow managed to remove the rest of this paragraph :(

The state field is not removed because I'm not a big fan of those
overloaded fields and I prefer to pay the 4 byte penalty for the
seperation.
Of course if there is the absolute requirement to reduce the size, I'm
not insisting on keeping it.

	tglx


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