linux-kernel - Re: [External] : Re: [REGRESSION][v6.17-rc1]sched/fair: Bump sd->max_newidle_lb

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Message-ID: <55869094-17ef-4291-b706-4600c919a0ea@os.amperecomputing.com>
Date: Fri, 31 Oct 2025 10:46:37 +0800
From: Adam Li <adamli@...amperecomputing.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Joseph Salisbury <joseph.salisbury@...cle.com>, Chris Mason
 <clm@...a.com>, clm@...com, Vincent Guittot <vincent.guittot@...aro.org>,
 Juri Lelli <juri.lelli@...hat.com>, Ingo Molnar <mingo@...hat.com>,
 dietmar.eggemann@....com, Steven Rostedt <rostedt@...dmis.org>,
 bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com,
 LKML <linux-kernel@...r.kernel.org>
Subject: Re: [External] : Re: [REGRESSION][v6.17-rc1]sched/fair: Bump
 sd->max_newidle_lb_cost when newidle balance fails

On 10/30/2025 5:30 PM, Peter Zijlstra wrote:
> 
> Hi Adam,
> 
> On Thu, Oct 30, 2025 at 03:22:34PM +0800, Adam Li wrote:
> 
>> Commit 155213a2aed4 brings ~6% regression for SpecJBB critical-jOPS on
>> AmpereOne server. 
>>
>> Baseline: v6.17 kernel + patch. Compared with baseline:
>> NI_TARGET	+7%
>> NI_SPARE	-3%
>> NI_RUNNABLE	-4%
>> NI_RANDOM	-3%
>>
>> So NI_TARGET reverts the performance regression.
>> The other options brings more regression for SpecJBB.
> 
> Excellent, how does NI_TARGET+NI_RANDOM work for you? Like Chris, I
> found that the schbench workload was really suffering from doing too
> many idle balance attempts that weren't really working out.
> 
> NI_RANDOM reduces the number of newidle balances based on the success
> ratio. Eg. if you have successful balancing 25% of the time, it will do
> 1 in 4 balances and count a successful balance as 4 to keep the
> accounting straight.
> 
> So workloads with a high success rate will keep newidle balance calls
> frequent, while workloads with a low success rate (like schbench) will
> greatly reduce the number of calls.

Please see the test result of SpecJBB critical-jOPS on AmpereOne:

Baseline: v6.17+patch
NI_TARGET:		+6%
NI_TARGET+NI_RANDOM:	+5%
NI_TARGET+NI_SPARE:	+4%
NI_TARGET+NI_RUNNABLE:	+2%

"NI_TARGET+NI_RANDOM" and "NI_TARGET" are very close.
I will do more test.

Thanks,
-adam