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

[Jes S. <je...@sg...>]

Zhang, Xiantao wrote:
>>From 697d50286088e98da5ac8653c80aaa96c81abf87 Mon Sep 17 00:00:00 2001
> From: Xiantao Zhang <xia...@in...>
> 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 <xia...@in...>

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