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

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Message-ID: <Z9hW_3cPN8u7VURV@slm.duckdns.org>
Date: Mon, 17 Mar 2025 07:08:15 -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, Joel.

On Mon, Mar 17, 2025 at 04:28:02AM -0400, Joel Fernandes wrote:
> Consider that the previous CPU is cache affined to the waker's CPU and
> is idle. Currently, scx's default select function only selects the
> previous CPU in this case if WF_SYNC request is also made to wakeup on the
> waker's CPU.
> 
> This means, without WF_SYNC, the previous CPU being cache affined to the
> waker and is idle is not considered. This seems extreme. WF_SYNC is not
> normally passed to the wakeup path outside of some IPC drivers but it is
> very possible that the task is cache hot on previous CPU and shares
> cache with the waker CPU. Lets avoid too many migrations and select the
> previous CPU in such cases.

Hmm.. if !WF_SYNC:

1. If smt, if prev_cpu's core is idle, pick it. If not, try to pick an idle
   core in widening scopes.

2. If no idle core is foudn, pick prev_cpu if idle. If not, search for an
   idle CPU in widening scopes.

So, it is considering prev_cpu, right? 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.

Isn't the cpus_share_cache() code block mostly about not doing
waker-affining if prev_cpu of the wakee is close enough and idle, so
waker-affining is likely to be worse?

Thanks.

-- 
tejun