Re: [PATCH 1/1] mm: Prevent dereferencing non-allocated per_cpu variables

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 27 Nov 2007 15:22:56 -0800 (PST)
Christoph Lameter <clameter@sgi.com> wrote:

> On Tue, 27 Nov 2007, Andrew Morton wrote:
> 
> > The prefetch however might still need some work - we can indeed do
> > prefetch() against a not-possible CPU's memory here.  And I do recall that
> > 4-5 years ago we did have a CPU (one of mine, iirc) which would oops when
> > prefetching from a bad address.  I forget what the conclusion was on that
> > matter.
> > 
> > If we do want to fix the prefetch-from-outer-space then we should be using
> > cpu_isset(cpu, *cpumask) here rather than cpu_possible().
> 
> Generally the prefetch things have turned out to be not that useful. How 
> about dropping the prefetch? I kept it because it was there.

I don't recall anyone ever demonstrating that prefetch is useful in-kernel.

I think I've heard of situations where benefits have been seen in userspace
- if a loop does a lot of calculation on each datum which it fetches then
there's a good opportunity to pipeline the fetch with the on-core
crunching.  But kernel doesn't do that sort of thing..