linux-kernel - Re: [PATCH v2] sched/fair: Prefer cache locality for EAS wakeup

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Message-ID: <a27b7500-0d4d-47b2-846d-b382d0c1c16d@arm.com>
Date: Sun, 2 Nov 2025 10:06:45 +0000
From: Christian Loehle <christian.loehle@....com>
To: Shubhang Kaushik OS <Shubhang@...amperecomputing.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>,
 Shubhang Kaushik <sh@...two.org>,
 Shijie Huang <Shijie.Huang@...erecomputing.com>,
 Frank Wang <zwang@...erecomputing.com>
Cc: Christopher Lameter <cl@...two.org>, Adam Li
 <adam.li@...erecomputing.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sched/fair: Prefer cache locality for EAS wakeup

On 10/31/25 18:11, Christian Loehle wrote:
> On 10/31/25 16:59, Shubhang Kaushik OS wrote:
>> Yes, I agree that EAS approach is not suitable in this case as they require a heterogenous CPU topology.
>> The issue is that the existing checks are for a completely idle CPU, whereas `cpu_overutilized` implies
>> the CPU is busy but not yet overloaded. I ventured into EAS as this `cpu_overutilized` relies on
>> `sched_energy_enabled()` being active. The point I wanted to convey is that - we still need a `cpu_busy?`
>> check to make use of the cache locality - for SMP systems. Would appreciate some pointers on the same lines..
> 
> So the main issue is that with existing code if a CPU is "overloaded" isn't all that well defined.
> For EAS we know if !rd->overloaded => all CPUs are !cpu_overutilized(). We could just pick any where

rd->overutilized is what I meant of course...
Hopefully the point is still clear, with EAS (and CAS given some limitations) we can answer the "is this
CPU _busy_?" question by just looking at that CPU.