Re: [PATCHSET sched_ext/for-7.4] sched_ext: Add NUMA balancing support

[Tejun Heo <tj@kernel.org>]

Hello, Andrea.

On Sun, Oct 04, 2026 at 09:27:06AM +0200, Andrea Righi wrote:
> This series lets a BPF scheduler opt into the NUMA hinting-fault scan with
> SCX_OPS_NUMA_BALANCING. The existing scan is driven from the sched_ext tick for
> the tasks of such a scheduler, and the hinting faults then work for them as they
> do for fair tasks: the kernel maintains their NUMA fault statistics and
> preferred node, and migrates the memory a task accesses toward the node it runs
> on. The tasks themselves are not migrated: their placement is left to the BPF
> scheduler.
>
> The resulting preferred node is exposed to BPF schedulers with
> scx_bpf_task_numa_nid(), which they can use to keep a task close to its memory
> when placing or balancing it. The value is advisory; nothing in the kernel acts
> on it for these tasks.

Is there a reason to gate this on a separate ops flag? An alternative would be
an op which is called when a task's preferred node changes, with the initial
node reported on enable like ops.set_weight(), and enabling the hinting-fault
scan iff the op is implemented. The scheduler then gets an event it can act on
rather than a value to poll. The node has a single writer, sched_setnuma(),
which already dequeues and re-enqueues the task under the rq lock, so the op
can be called from there the same way ops.set_weight() is called from the
reweight path.

Thanks.

-- 
tejun