linux-kernel - Re: [REGRESSION] sched/fair: Reimplement NEXT

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Message-ID: <756efd17-682f-4ffc-b8d9-dbb2517bc152@arm.com>
Date: Fri, 2 Jan 2026 16:52:53 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Ryan Roberts <ryan.roberts@....com>,
 Mel Gorman <mgorman@...hsingularity.net>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
 Aishwarya TCV <Aishwarya.TCV@....com>
Subject: Re: [REGRESSION] sched/fair: Reimplement NEXT_BUDDY to align with
 EEVDF goals

On 02.01.26 13:38, Ryan Roberts wrote:
> Hi, I appreciate I sent this report just before Xmas so most likely you haven't
> had a chance to look, but wanted to bring it back to the top of your mailbox in
> case it was missed.
> 
> Happy new year!
> 
> Thanks,
> Ryan
> 
> On 22/12/2025 10:57, Ryan Roberts wrote:
>> Hi Mel, Peter,
>>
>> We are building out a kernel performance regression monitoring lab at Arm, and 
>> I've noticed some fairly large perofrmance regressions in real-world workloads, 
>> for which bisection has fingered this patch.
>>
>> We are looking at performance changes between v6.18 and v6.19-rc1, and by 
>> reverting this patch on top of v6.19-rc1 many regressions are resolved. (We plan 
>> to move the testing to linux-next over the next couple of quarters so hopefully 
>> we will be able to deliver this sort of news prior to merging in future).
>>
>> All testing is done on AWS Graviton3 (arm64) bare metal systems. (R)/(I) mean 
>> statistically significant regression/improvement, where "statistically 
>> significant" means the 95% confidence intervals do not overlap".

You mentioned that you reverted this patch 'patch 2/2 'sched/fair:
Reimplement NEXT_BUDDY to align with EEVDF goals'.

Does this mean NEXT_BUDDY is still enabled, i.e. you haven't reverted
patch 1/2 'sched/fair: Enable scheduler feature NEXT_BUDDY' as well?

---

Mel mentioned that he tested on a 2-socket machine. So I guess something
like my Intel Xeon Silver 4314:

cpu0 0 0
domain0 SMT 00000001,00000001
domain1 MC 55555555,55555555
domain2 NUMA ffffffff,ffffffff

node distances:
node   0   1
  0:  10  20
  1:  20  10

Whereas I assume the Graviton3 has 64 CPUs (cores) flat in a single MC
domain? I guess topology has influence in benchmark numbers here as well.

---

There was also a lot of improvement on schbench (wakeup latency) on
higher percentiles (>= 99.0th) on the 2-socket machine with those 2
patches. I guess you haven't seen those on Grav3?

[...]