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: <15b871f2-49b8-4b62-926d-31f93ad49f51@intel.com>
Date: Thu, 16 Oct 2025 00:25:27 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Peter Zijlstra <peterz@...radead.org>, 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>, 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>, 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 10/15/2025 7:04 PM, Peter Zijlstra wrote:
> 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.

SCHED_CACHE allows user space to turn on/off the feature at runtime,
while sched_cache_allow is a hardware capability. This capability is
  disabled if there are no multiple LLCs within one node. I’m not sure
if using one key could support the above two scenarios.

thanks,
Chenyu