Re: [rfc] forked kernel task and mm structures imbalanced on NUMA

Nick Piggin
2010-06-01T15:59:50Z

On Tue, Jun 01, 2010 at 05:48:10PM +0200, Andi Kleen wrote:
> Nick Piggin <npiggin@suse.de> writes:
>
> > This isn't really a new problem, and I don't know how important it is,
> > but I recently came across it again when doing some aim7 testing with
> > huge numbers of tasks.
>
> Seems reasonable. Of course you need to at least
> save/restore the old CPU policy, and use a subset of it.

The mpolicy? My patch does that (mpol_prefer_cpu_start/end). The real
problem is that it can actually violate the parent's mempolicy. For
example MPOL_BIND and cpus_allowed set on a node outside the mempolicy.

What is needed is to execute with the existing mempolicy, but from
the point of view of the destination CPU. A bit more work on the
mpol code is required, but this is good enough for basic tests.

> Another approach would be to migrate this on touch, but that is probably
> slightly more difficult. The advantage would be that on multiple
> migrations it would follow. And it would be a bit slower for
> the initial case.

Migrate what on touch? Talking mainly about kernel memory structures,
task_struct, mm, vmas, page tables, kernel stack, etc.