Re: Incorrect CPU process accounting using CONFIG_HZ=100

[Pavel Machek <pavel@suse.cz>]

Hi!

> >>>Wouldn't merging the two approaches be in the interest of conserving 
> >>>cpu resources, while at the same time reflecting an accurate view of 
> >>>cpu utilization?
> >>
> >>I think that this would be a worthwhile endeavour once/if 
> >>sched_clock() is fixed.  This is especially the case as CPUs get 
> >>faster as many tasks may run to completion in less than a tick.
> >
> >That may not be as simple as it seems. To properly account system v user 
> >time using the sched_clock we'd have to hook into arch dependant asm 
> >code to know when entering and exiting kernel context. That is far more 
> >invasive than the simple on/off runqueue timing we currently do for 
> >scheduling accounting.
> 
> Yes, it is a problem and we may have to do something approximate like 
> counting ticks for sys time and subtracting that from the total to get 
> user time when reporting the times to user space (only a bit more 
> complex to make sure we don't end up with negative times).

Yes, please.

> How is it intended to handle this problem in the tickless kernel?

Even our "tickless" kernels do tick while CPU is busy.
								Pavel
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