Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt

Russell King
2005-07-14T19:15:25Z

On Thu, Jul 14, 2005 at 10:21:41AM -0700, Linus Torvalds wrote:
> In other words, the _right_ way to do this is literally
>
> unsigned long timeout = jiffies + HZ/2;
> for (;;) {
> if (ready())
> return 0;
> if (time_after(timeout, jiffies))
> break;
> msleep(10);
> }
>
> which is unquestionably more complex, yes, but it's more complex because
> it is CORRECT!

Umm. Except, according to your description of what it's supposed to
do, the above code can have an accumulating error.

unsigned long timeout, max_timeout, now;

now = jiffies;
timeout = now + HZ/100;
max_timeout = now + HZ/2;

for (;;) {
if (ready())
return 0;
if (time_after(timeout, jiffies))
break;
sleep_until(timeout);
timeout += HZ/100;
}

would be even more correct, if we had an absolute schedule_timeout()
called sleep_until(). If we woke up late, we will schedule the next
wakeup exactly 10ms after the previous wakeup _should_ have happened.

Quick! Convert all the kernel interfaces back to absolute time! 8)

--
Russell King
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
maintainer of: 2.6 Serial core