linux-kernel - Re: [PATCH 04/19] sched/fair: Introduce a static key to enable cache aware only for multi LLCs

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Message-ID: <20251015110450.GO3289052@noisy.programming.kicks-ass.net>
Date: Wed, 15 Oct 2025 13:04:50 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Ingo Molnar <mingo@...hat.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	"Gautham R . Shenoy" <gautham.shenoy@....com>,
	Chen Yu <yu.c.chen@...el.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>,
	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>,
	Aubrey Li <aubrey.li@...el.com>, Zhao Liu <zhao1.liu@...el.com>,
	Chen Yu <yu.chen.surf@...il.com>, Libo Chen <libo.chen@...cle.com>,
	Adam Li <adamli@...amperecomputing.com>,
	Tim Chen <tim.c.chen@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/19] sched/fair: Introduce a static key to enable cache
 aware only for multi LLCs

On Sat, Oct 11, 2025 at 11:24:41AM -0700, Tim Chen wrote:
> From: Chen Yu <yu.c.chen@...el.com>
> 
> Enable cache-aware load balancing only if at least 1 NUMA node has
> more than one LLC.
> 
> Suggested-by: Libo Chen <libo.chen@...cle.com>
> Suggested-by: Adam Li <adamli@...amperecomputing.com>
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> ---
>  kernel/sched/fair.c     | 15 ++++++++++++---
>  kernel/sched/sched.h    |  1 +
>  kernel/sched/topology.c | 14 ++++++++++++--
>  3 files changed, 25 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cd080468ddc9..3d643449c48c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1208,6 +1208,14 @@ static s64 update_se(struct rq *rq, struct sched_entity *se)
>  __read_mostly unsigned int llc_overload_pct       = 50;
>  __read_mostly unsigned int llc_imb_pct            = 20;
>  
> +DEFINE_STATIC_KEY_FALSE(sched_cache_allowed);
> +
> +static inline bool sched_cache_enabled(void)
> +{
> +	return sched_feat(SCHED_CACHE) &&
> +		static_branch_likely(&sched_cache_allowed);
> +}

Urgh; do we really need _2_ static keys stacked for this? I'm thinking
one should be well enough.