Re: [PATCHSET v2 sched_ext/for-7.5] sched_ext: Add NUMA balancing support

[Peter Zijlstra]

On Tue, Oct 27, 2026 at 12:38:49PM +0100, Andrea Righi wrote:
> On Tue, Oct 27, 2026 at 11:52:09AM +0100, Peter Zijlstra wrote:
> > On Tue, Oct 27, 2026 at 05:13:29AM +0100, Andrea Righi wrote:
> > 
> > > Ideally the ultimate goal is to give BPF schedulers both controls that
> > > NUMA balancing combines: moving a task to the node where its memory is
> > > and moving the memory a task uses to the node where the task runs. This
> > > series provides the first: with the preferred node available, a
> > > scheduler can decide where to run the task. A second series, to follow
> > > once this one lands, will provide the second: a kfunc that lets a
> > > scheduler request the node a task's memory should migrate to.
> > 
> > Fair has no real control over the memory migration part either; why
> > would you want to give BPF that?
> 
> Yeah, that's a fair point and the wording in the cover letter is a bit
> too vague.
> 
> The use case behind the second part is accelerator/device locality,
> discussed at LPC in the sched_ext MC [1]: with multi-GPU systems the
> heaviest user of a task's host memory is often the device, which NUMA
> balancing can't see, since hinting faults only track CPU accesses. So
> memory follows the CPU even when it should stay close to the device, and
> today the only option is to pin everything with numactl and/or disable
> NUMA balancing.
> 
> But that needs to be discussed separately: as Gregory pointed out at
> LPC, NUMA balancing needs some cleanups first, and the right interface
> may not be BPF-specific at all (e.g., extending the home node concept of
> set_mempolicy_home_node() to NUMA balancing, or a device-aware hint in
> mm that both fair and ext could use, or something else).
> 
> The first step for now is to, at least, allow sched_ext to see the
> preferred node computed by NUMA balancing, which is what this series
> does.

Right, I think that came up at OSPM as well; regular numa balancing
would want this as well. This really should not be a bpf thing.