Re: [PATCH 4/4] sched/fair: Proportional newidle balance

[Mohamed Abuelfotoh, Hazem]

On 27/01/2026 09:13, Peter Zijlstra wrote:
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> On Tue, Jan 27, 2026 at 09:50:25AM +0100, Peter Zijlstra wrote:
> > On Sun, Jan 25, 2026 at 12:22:21PM +0000, Mohamed Abuelfotoh, Hazem wrote:
> >
> > > I can confirm that we are seeing a 4-11% performance regression in v6.12.66
> > > on multiple benchmarks running on c7a.4xlarge AWS EC2 instances that are
> > > powered by AMD EPYC 9R14-series CPU (code-named Genoa) and c7i.4xlarge which
> > > is powered by 4th-Generation Intel Xeon Scalable processor (code-named
> > > Sapphire Rapids). The regression is caused by the commit 33cf66d88306
> > > ("sched/fair: Proportional newidle balance"). We were able to reclaim the
> > > performance back after reverting this commit. We also noticed that the
> > > impact is higher on AMD vs Intel.
> > >
> > > Benchmark Name |  Description                                   | Unit
> > > postgresql     |  HammerDB workload (TPC-C-like benchmark)  | NOPM
> > > nginx_lb       |  Testing NGINX as a load balancer      | RPS
> > > memcached      |  Testing using Lancet load generator       | QPS
> > >
> > > **Results on v6.12.66**
> > >
> > > Benchmark name | SUT EC2 Instance | Regression percentage
> > > postgresql     | c7a.4xlarge      | -4.0%
> > > postgresql     | c7i.4xlarge      | -4.0%
> > > nginx_lb       | c7a.4xlarge      | -5.0%
> > > memcached      | c7a.4xlarge      | -11.0%
> >
> > So only postgres has a regression on Intel? Memcached doesn't show
> > anything?
>
> And just to be sure, v6.12.43-v6.12.65 have no problem?
>
> That is, afaict those are the kernels that have:
>
>    fc4289233e4b sched/fair: Bump sd->max_newidle_lb_cost when newidle balance fails
>
> But not yet have:
>
>    1b9c118fe318 sched/fair: Proportional newidle balance
>    c6ae271bc5fd sched/fair: Small cleanup to update_newidle_cost()
>    52aa889c6f57 sched/fair: Small cleanup to sched_balance_newidle()
>    81343616e712 sched/fair: Revert max_newidle_lb_cost bump
>
> Because fc4289233e4b was also causing a ton of regressions (but also
> improving some workloads). 81343616e712 then reverts this and
> 1b9c118fe318 is supposed to be a compromise between these two.
>
> So if your workloads are not affected by fc4289233e4b and 81343616e712,
> but somehow 1b9c118fe318 is causing fail, then I'm a little puzzled.
We have definitely seen significant performance regression specifically 
on DB workloads because of fc4289233e4b ("sched/fair: Bump 
sd->max_newidle_lb_cost when newidle balance fails") which we reported 
in [1]. We were able to reclaim the performance back with ("81343616e712 
sched/fair: Revert max_newidle_lb_cost bump") before we start seeing 
negative impact from 1b9c118fe318 sched/fair: Proportional newidle balance.



[1] 
https://lore.kernel.org/all/006c9df2-b691-47f1-82e6-e233c3f91faf@xxxxxxxxxx/T/#mb96105e4a320659b5aa68ec112bbeafaae37e769