linux-kernel - Re: [PATCH RFC] sched_ext: Choose prev_cpu if idle and cache affine without WF

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Message-ID: <Z9ihbiUV7dohm7FT@slm.duckdns.org>
Date: Mon, 17 Mar 2025 12:25:50 -1000
From: Tejun Heo <tj@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: linux-kernel@...r.kernel.org, David Vernet <void@...ifault.com>,
	Andrea Righi <arighi@...dia.com>,
	Changwoo Min <changwoo@...lia.com>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH RFC] sched_ext: Choose prev_cpu if idle and cache affine
 without WF_SYNC

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