linux-kernel - Re: [PATCH 19/19] sched/fair: Add user control to adjust the tolerance of cache-aware scheduling

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Message-ID: <32fe2f56-c479-4679-a9a5-12373c544016@intel.com>
Date: Wed, 29 Oct 2025 20:54:14 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Aaron Lu <ziqianlu@...edance.com>, Tim Chen <tim.c.chen@...ux.intel.com>
CC: Peter Zijlstra <peterz@...radead.org>, 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 19/19] sched/fair: Add user control to adjust the
 tolerance of cache-aware scheduling

On 10/29/2025 4:07 PM, Aaron Lu wrote:
> On Sat, Oct 11, 2025 at 11:24:56AM -0700, Tim Chen wrote:
> ... ...
>> +static inline int get_sched_cache_cap_scale(void)
>> +{
>> +	return (llc_overload_pct / cpu_smt_num_threads);
>> +}
>> +
> ... ...
>> @@ -9749,7 +9811,7 @@ static inline int task_is_ineligible_on_dst_cpu(struct task_struct *p, int dest_
>>    * (default: ~50%)
>>    */
>>   #define fits_llc_capacity(util, max)	\
>> -	((util) * 100 < (max) * llc_overload_pct)
>> +	((util) * 100 < (max) * get_sched_cache_cap_scale())
>>
> 
> With this change, fits_llc_capacity() would be false if util is just 1/4
> of max(which is llc's total cpu capacity), is this intended?

Yes, it was changed to this because we want to avoid performance 
regressions
in some systems with a large number of SMTs per core-aggressive task 
aggregation
  is harmful to those systems. However, upon further thought, since we 
have a
user-space knob to control how aggressively users want to enable task 
aggregation,
we can try removing cpu_smt_num_threads and let users decide. I'll do some
tests to check the impact.

thanks,
Chenyu