linux-kernel - update on hrtimer based select/poll and range-hrtimers

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Date:	Sun, 7 Sep 2008 11:11:37 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...e.hu, tglx@...x.de, torvalds@...ux-foundation.org,
	peterz@...radead.org
Subject: update on hrtimer based select/poll and range-hrtimers

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....

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

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