linux-kernel - Re: [RFC PATCH v4 07/28] sched: Add helper function to decide whether to allow cache aware scheduling

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Message-ID: <20251001131715.GO4067720@noisy.programming.kicks-ass.net>
Date: Wed, 1 Oct 2025 15:17:15 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Chen Yu <yu.c.chen@...el.com>
Cc: Ingo Molnar <mingo@...hat.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	"Gautham R . Shenoy" <gautham.shenoy@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	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>,
	Libo Chen <libo.chen@...cle.com>,
	Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
	Hillf Danton <hdanton@...a.com>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	Jianyong Wu <jianyong.wu@...look.com>,
	Yangyu Chen <cyy@...self.name>,
	Tingyin Duan <tingyin.duan@...il.com>,
	Vern Hao <vernhao@...cent.com>, Len Brown <len.brown@...el.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Aubrey Li <aubrey.li@...el.com>, Zhao Liu <zhao1.liu@...el.com>,
	Chen Yu <yu.chen.surf@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 07/28] sched: Add helper function to decide
 whether to allow cache aware scheduling

On Sat, Aug 09, 2025 at 01:03:10PM +0800, Chen Yu wrote:
> From: Tim Chen <tim.c.chen@...ux.intel.com>
> 
> Cache-aware scheduling is designed to aggregate threads into their
> preferred LLC, either via the task wake up path or the load balancing
> path. One side effect is that when the preferred LLC is saturated,
> more threads will continue to be stacked on it, degrading the workload's
> latency. A strategy is needed to prevent this aggregation from going too
> far such that the preferred LLC is too overloaded.

So one of the ideas was to extend the preferred llc number to a mask.
Update the preferred mask with (nr_threads / llc_size) bits, indicating
the that many top llc as sorted by occupancy.