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

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Message-Id: <200703260811.18236.a1426z@gawab.com>
Date:	Mon, 26 Mar 2007 08:11:18 +0300
From:	Al Boldi <a1426z@...ab.com>
To:	linux-kernel@...r.kernel.org
Subject: Re: [patch] sched: accurate user accounting

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!

--
Al

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