Re: 2.5.59-mm2

[Arjan van de Ven <arjanv@redhat.com>]

On Sun, Jan 19, 2003 at 11:45:35AM -0800, Nakajima, Jun wrote:
> We initially implemented it in user level, accessing /proc/interrupts. We have two issues/concerns at that point. And we saw better results with kernel mode.

> - the data structures required, such as kstat, are already in the kernel
>   and converting the text info from /proc/interrupts was costly in
>   user mode.

costly is a relative thing. a dozen cycles perhaps; do it once per
10 seconds and it's invisbile. I agree that if you want to do it thousands
of times per second it might become a problem.But so far I don't see the
real need for that.

> - we suspect that frequent writes (asynchronous to interrupts)
>   to /proc/irq/N/smp_affinity might expose a race condition in interrupt
>   machinery. For example, we saw a hang caused by such a write.

if there's a bug there it needs fixing anyway; even inside the kernel
you'll have a similar race I suspect

> So to implement it in user level efficiently, we need API that
> - that provide binary data that can be easily processed by such a daemon,

there is rightfully a veto on such ABI and it's also not needed.
/proc/interrupts is less than 4Kb normally; it'll be in cache so parsing
it will be cheap. Sure the code I posted isn't optimal (far from it) but
that can be optimized a lot.

Greetings,
  Arjan van de Ven
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