Re: [PATCH 1/1] mm: Prevent dereferencing non-allocated per_cpuvariables
From: Andrew Morton
Date: Tue Nov 27 2007 - 18:42:49 EST
On Tue, 27 Nov 2007 15:22:56 -0800 (PST)
Christoph Lameter <clameter@xxxxxxx> 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..
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