Re: [02/17][PATCH] Implement smp_call_function_mask for ia64 - V8

Jeremy Fitzhardinge
2008-03-31T15:02:22Z

Jes Sorensen wrote:
> I'm a little wary of the performance impact of this change. Doing a
> cpumask compare on all smp_call_function calls seems a little expensive.
> Maybe it's just noise in the big picture compared to the actual cost of
> the IPIs, but I thought I'd bring it up.
>
> Keep in mind that a cpumask can be fairly big these days, max NR_CPUS
> is currently 4096. For those booting a kernel with NR_CPUS at 4096 on
> a dual CPU machine, it would be a bit expensive.
>

Unless your hardware has remarkably fast IPIs, I think really the cost
of scanning 512 bytes is going to be in the noise...

This change has been on the x86 side for ages, and not even Ingo made a
peep about it ;)

> Why not keep smp_call_function() the way it was before, rather than
> implementing it via the call to smp_call_function_mask()?
>

Because Xen needs a different core implementation (because of its
different IPI implementation), and it would be better to just have to do
one of them rather than N.

J