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SubjectRe: [rfc][patch] x86-64 new smp_call_function design

* Nick Piggin <npiggin@suse.de> wrote:

> On Wed, Feb 27, 2008 at 04:02:10PM +0100, Ingo Molnar wrote:
> >
> > * Nick Piggin <npiggin@suse.de> wrote:
> >
> > > > the two structures are quite similar in size and role - why not have
> > > > a type field and handle them largely together? I think we should try
> > > > to preserve a single queue and a single vector - that would remove a
> > > > number of ugly special-cases from the patch.
> > >
> > > A single queue will kill one of the big fundamental scalability
> > > improvements of the call_single. That's the problem.
> >
> > hm, indeed. Then how about the other way around: couldnt the normal
> > all-cpus SMP function call be implemented transparently via using
> > smp_call_single() calls?
>
> That's possible, but it is slower and less scalable on my 8-way, and
> I suspect it might become even slower than the generic code on larger
> systems.

i dont mean "implement call-all as a series of call-single" calls, but
use just a single queue of requests and differentiate on the data
structure level. Right now you use the vector # as the differentiator.

but ... no strong feelings, i'm just playing the devil's advocate :)
Your work is great (and i now see that i forgot to state this clearly
enough in my first mail - i thought i to be obvious, based on your
numbers!), i'm really just trying to micro-optimize the concept.

Could you try to unify it with the 32-bit code, preferably into a
separate, unified arch/x86/kernel/smp.c file? Such an approach would
make it into x86.git in a heartbeat :)

> > The vector duplication is really ugly and feels wrong.
>
> Why?

it's ~0.5% of our irq vector space? :-)

we could also try to implement a "NOP" type of single call [ using a nop
callback is one of the easiest possibilities there ;) ] - which would
allow us to eliminate the special reschedule vector as well. That means
we could consolidate all the SMP cross-calls into a single vector.

OTOH, i see how you save multiplexing/demultiplexing complexity on both
the sending and the receiving side by using the separate vectors. So i
guess, if it's fast enough, we should indeed do your two vectors
approach and also merge the reschedule special vector into the
single-call path and thus have no effect on the size of the vector
space. (no need to add a new vector even - just rename the reschedule
vector to single-call vector)

Ingo


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