Re: [PATCH 01/13] hrtimer: round up relative start time

[Roman Zippel <zippel@linux-m68k.org>]

Hi,

On Mon, 13 Feb 2006, Thomas Gleixner wrote:

> This adds an artificial offset to the expiry time, for what reason? The
> expiry code makes sure that timers can not expire early. See:
> 
> 	timer = rb_entry(node, struct hrtimer, node);
> 	if (now.tv64 <= timer->expires.tv64)
> 		break;
> 
> in kernel/hrtimers.c:run_hrtimer_queue(), where now is already tick
> aligned.
> 
> Please provide a testcase (or detailed use-case) which proves that this
> is necessary.

Let's assume a get_time() which simply returns xtime and so has a 
resolution of around TICK_NSEC. This means the real time when one calls 
get_time() is somewhere between xtime and xtime+TICK_NSEC. Assuming the 
real time is xtime+TICK_NSEC-1, get_time() will return xtime and a 
relative timer with TICK_NSEC-1 will expire immediately.
The old code did this correctly. For most hardware this is not a real 
issue, as the delivery time is larger than the clock resolution, but 
unless you can guarantee it's not an issue on _any_ supported hardware, 
this fix is needed. As I already said this can be better fixed as soon as 
we have a better clock abstraction, until then this is only restores the 
old behaviour.

bye, Roman
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