Re: It hurts when I shoot myself in the foot

[Kasper Dupont <kasperd@daimi.au.dk>]

Alan Cox wrote:
> 
> > Couldn't that be solved in one of the following ways?
> >
> > 1) Disable pre-emption while reading TSC and CPU nr.
> > 2) Use affinity for processes pre-empted in kernel mode.
> > 3) Disable pre-emption for SMP systems.
> 
> You can solve it by disabling pre-emption (and given its questionable
> value doing so permanently might not be a bad idea).

Questionable value of what? TSC or preemption?

> However if you simply
> disable pre-emption during udelay() calls then you've just screwed yourself
> by removing 99% of the use pre-emption had.

I wouldn't want to disable preemption during udelays.
Either I would disable and enable preemption on every
pass through the loop. Or I would just manually check
for every pass if I should give up the CPU. This
obviously requires more computation for every pass,
but being a busy waiting loop I don't see a problem.

Otherwise I would lock the process to a fixed CPU for
the duration of udelay.

> 
> Given all the pain its probably better to not use the TSC

Do we have better alternatives for high resolution
time meassurements?

-- 
Kasper Dupont -- der bruger for meget tid på usenet.
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