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

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Message-Id: <1220820399.8687.103.camel@twins.programming.kicks-ass.net>
Date:	Sun, 07 Sep 2008 22:46:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, tglx@...x.de,
	torvalds@...ux-foundation.org
Subject: Re: update on hrtimer based select/poll and range-hrtimers

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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