linux-kernel - Re: CPU load

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Date:	Wed, 14 Feb 2007 19:09:09 +1100
From:	Con Kolivas <kernel@...ivas.org>
To:	malc <av1474@...tv.ru>
Cc:	Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org
Subject: Re: CPU load

On Wednesday 14 February 2007 18:28, malc wrote:
> On Wed, 14 Feb 2007, Con Kolivas wrote:
> > On Wednesday 14 February 2007 09:01, malc wrote:
> >> On Mon, 12 Feb 2007, Pavel Machek wrote:
> >>> Hi!
>
> [..snip..]
>
> >>> I have (had?) code that 'exploits' this. I believe I could eat 90% of
> >>> cpu without being noticed.
> >>
> >> Slightly changed version of hog(around 3 lines in total changed) does
> >> that easily on 2.6.18.3 on PPC.
> >>
> >> http://www.boblycat.org/~malc/apc/load-hog-ppc.png
> >
> > I guess it's worth mentioning this is _only_ about displaying the cpu
> > usage to userspace, as the cpu scheduler knows the accounting of each
> > task in different ways. This behaviour can not be used to exploit the cpu
> > scheduler into a starvation situation. Using the discrete per process
> > accounting to accumulate the displayed values to userspace would fix this
> > problem, but would be expensive.
>
> Guess you are right, but, once again, the problem is not so much about
> fooling the system to do something or other, but confusing the user:

Yes and I certainly am not arguing against that.

>
> a. Everything is fine - the load is 0%, the fact that the system is
>     overheating and/or that some processes do not do as much as they
>     could is probably due to the bad hardware.
>
> b. The weird load pattern must be the result of bugs in my code.
>     (And then a whole lot of time/effort is poured into fixing the
>      problem which is simply not there)
>
> The current situation ought to be documented. Better yet some flag can
> be introduced somewhere in the system so that it exports realy values to
> /proc, not the estimations that are innacurate in some cases (like hog)

I wouldn't argue against any of those either. schedstats with userspace tools 
to understand the data will give better information I believe.

-- 
-ck
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