Re: [PATCH RFC] sched_ext: Choose prev_cpu if idle and cache affine without WF_SYNC

[Tejun Heo]

Hello,

On Mon, Mar 17, 2025 at 11:07:36PM +0100, Joel Fernandes wrote:
...
> > I think it's preferring idle core a
> > bit too much - it probably doesn't make sense to cross the NUMA boundary if
> > there is an idle CPU in this node, at least.
> 
> Yes, that is a bit extreme. I believe that is what my patch is fixing.  If
> previous CPU and current CPU share cache, we prefer busy cores with free
> previous idle SMT, otherwise go looking for fully idle cores. But it sounds like
> from Peter's reply that is not necessarily a good thing to do in case 'fast
> numa'. So I guess there is no good answer (?) or way of doing it.

Yeah, recent AMD CPUs can be configured into NUMA mode where each chiplet is
reported as a node and they do behave like one as each has its own memory
connection but the distances among them are significantly closer than
traditional multi-socket. That said, I think the implementation is still a
bit too happy to jump the boundary. What Andrea is suggesting seems
reasonable?

Thanks.

-- 
tejun