Re: update on hrtimer based select/poll and range-hrtimers

From: Peter Zijlstra
Date: Sun Sep 07 2008 - 16:47:11 EST


On Sun, 2008-09-07 at 11:11 -0700, Arjan van de Ven wrote:
> Hi,
>
> since the last lkml posting I've merged a few fixes and added comments
> from Peter, and I've redone the "estimate_accuracy" function.
>
> Rather than reposting the entire series, I'll point to the git tree at
>
> git://git.kernel.org/pub/scm/linux/kernel/git/arjan/linux-2.6-hrtimer.git
>
> and I've pasted the new function below.
> Rather than having the hardcoded steps from the "Linus" function, I've
> turned it into:
> 0 for realtime tasks
> "0.1% of the time" for not-nice, not realtime tasks
> "0.5% of the time" for nice, not realtime tasks
> with a cap of 100msec for both.
>
> I would like to request feedback on this approach; I think this is
> better than the "hardcoded steps" as before, but maybe someone can come
> up with an ever better idea....

logarithms pop to mind (again :-)

> static unsigned long __estimate_accuracy(struct timespec *tv)
> {
> unsigned long slack;
> int divfactor = 1000;
>
> if (task_nice(current))

This triggers for both -nice and +nice tasks, it might be worth
differentiating between those.

> divfactor = divfactor / 5;
>
> slack = tv->tv_nsec / divfactor;
> slack += tv->tv_sec * (NSEC_PER_SEC/divfactor);
>
> if (slack > 100 * NSEC_PER_MSEC)
> slack = 100 * NSEC_PER_MSEC;
> return slack;
> }
>
> static unsigned long estimate_accuracy(struct timespec *tv)
> {
> unsigned long ret;
> struct timespec now;
>
> /*
> * Realtime tasks get a slack of 0 for obvious reasons.
> */
>
> if (current->policy == SCHED_FIFO ||
> current->policy == SCHED_RR)
> return 0;

rt_task(current) ?

> ktime_get_ts(&now);
> now = timespec_sub(*tv, now);
> ret = __estimate_accuracy(&now);
> if (ret < current->timer_slack_ns)
> return current->timer_slack_ns;

pull out the max from __estimate_accuracy() and use clamp() here?

> return ret;
> }


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