Re: [PATCH 5/6] sched_ext: Add a kfunc to read a task's preferred NUMA node

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 27, 2026 at 07:57:36AM +0100, Andrea Righi wrote:
> Hi Prateek,
> 
> On Tue, Oct 27, 2026 at 11:42:22AM +0530, K Prateek Nayak wrote:
> > Hello Andrea,
> > 
> > On 10/27/2026 9:43 AM, Andrea Righi wrote:
> > > +__bpf_kfunc s32 scx_bpf_task_numa_nid(const struct task_struct *p)
> > > +{
> > > +#ifdef CONFIG_NUMA_BALANCING
> > > +	if (task_on_scx(p)) {
> > > +		struct scx_sched *sch;
> > > +
> > > +		guard(rcu)();
> > > +		sch = scx_task_sched_rcu(p);
> > > +		if (!sch || !SCX_HAS_OP(sch, set_numa_node))
> > > +			return NUMA_NO_NODE;
> > > +	}
> > > +
> > > +	return READ_ONCE(p->numa_preferred_nid);
> > 
> > Would the ext scheduler need the raw numa_preferred_nid when
> > sched_numa_balancing is disabled?
> > 
> > It may just be pointing to some stale value if NUMA balancing
> > was disabled at runtime right?
> 
> Right, if NUMA balancing is disabled at runtime the value stays at the
> last preference.
> 
> This is consistent with fair: disabling NUMA balancing at runtime only
> flips the sched_numa_balancing static key, p->numa_preferred_nid is
> never reset, fair stops acting on it in most places (e.g.,
> migrate_degrades_locality()), but not everywhere, find_idlest_group()
> still uses it in the slow wakeup path. So a BPF scheduler sees the same
> value that fair keeps and can decide whether to use it or not.
> 
> A more accurate approach could be to notify the sched classes when
> sched_numa_balancing changes (e.g., resetting the preferred node of all
> tasks when it's disabled and recomputing it when it's re-enabled), but
> that would be a significant rework, involving fair as well, and it's
> outside the scope of this series. I'd rather start by making ext
> consistent with fair, and then rework both in a more accurate and
> complete way as a follow-up, if we want.

But should we not cleanup first and then add shiny on top?