Linux Virtualization - Mar 2008 - [02/17][PATCH] Implement smp_call_function_mask for ia64 - V8

Zhang, Xiantao wrote:
>>From 697d50286088e98da5ac8653c80aaa96c81abf87 Mon Sep 17 00:00:00 2001
> From: Xiantao Zhang <xiantao.zhang at intel.com>
> Date: Mon, 31 Mar 2008 09:50:24 +0800
> Subject: [PATCH] KVM:IA64: Implement smp_call_function_mask for ia64
> 
> This function provides more flexible interface for smp
> infrastructure.
> Signed-off-by: Xiantao Zhang <xiantao.zhang at intel.com>
Hi Xiantao,

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.

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

Cheers,
Jes

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