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

Nick Piggin
2010-06-01T16:31:09Z

On Tue, Jun 01, 2010 at 06:20:25PM +0200, Andi Kleen wrote:
> On Wed, Jun 02, 2010 at 01:59:43AM +1000, Nick Piggin wrote:
> > 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.
>
> I don't see where you store 'old', but maybe I missed it.

It's the argument returned by mpol_prefer_cpu_start. Yes this also
opens races for lost-write when we have concurrent mpol changes. So
I'm not claiming the code is right.

> > > 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.
>
> Migrate task_struct, mm, vmas, page tables, kernel stack
> on reasonable touch. As long as they are not shared it shouldn't
> be too difficult.

Possible but that's a lot further off (considering we don't even migrate
user memory) and is complimentary to this patch.