linux-kernel - Re: [patch] sched: accurate user accounting

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Date:	Mon, 26 Mar 2007 18:45:42 +1000
From:	Con Kolivas <kernel@...ivas.org>
To:	Al Boldi <a1426z@...ab.com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>, zwane@...radead.org
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [patch] sched: accurate user accounting

On Monday 26 March 2007 15:11, Al Boldi wrote:
> Con Kolivas wrote:
> > Ok this one is heavily tested. Please try it when you find the time.
>
> It's better, but still skewed.  Try two chew.c's; they account 80% each.
>
> > ---
> > Currently we only do cpu accounting to userspace based on what is
> > actually happening precisely on each tick. The accuracy of that
> > accounting gets progressively worse the lower HZ is. As we already keep
> > accounting of nanosecond resolution we can accurately track user cpu,
> > nice cpu and idle cpu if we move the accounting to update_cpu_clock with
> > a nanosecond cpu_usage_stat entry.
>
> That's great and much needed, but this is still probed; so what's wrong
> with doing it in-lined?
>
> > This increases overhead slightly but
> > avoids the problem of tick aliasing errors making accounting unreliable.
>
> Higher scheduling accuracy may actually offset any overhead incurred, so
> it's well worth it; and if it's in-lined it should mean even less overhead.
>
> > +       /* Sanity check. It should never go backwards or ruin accounting
> > */ +       if (unlikely(now < p->last_ran))
> > +               goto out_set;
>
> If sched_clock() goes backwards, why not fix it, instead of hacking around
> it?
>
>
> Thanks!

Actually I'm going to give up this idea as not worth my effort given the 
sched_clock fsckage that seems to cause so much grief. If someone else wants 
to take up the challenge feel free to.

Andrew please drop this patch. It's still broken and I have too much on my 
plate to try and debug it sorry.

-- 
-ck
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