linux-kernel - Re: [PATCH 0/6 v8] sched/fair: Add push task mechanism and handle more EAS cases

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Message-ID: <20260206183215.eqc2b7wspwhdruil@airbuntu>
Date: Fri, 6 Feb 2026 18:32:15 +0000
From: Qais Yousef <qyousef@...alina.io>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org,
	pierre.gondois@....com, kprateek.nayak@....com,
	hongyan.xia2@....com, christian.loehle@....com,
	luis.machado@....com
Subject: Re: [PATCH 0/6 v8] sched/fair: Add push task mechanism and handle
 more EAS cases

On 12/02/25 19:12, Vincent Guittot wrote:
> This is a subset of [1] (sched/fair: Rework EAS to handle more cases)
> 
> [1] https://lore.kernel.org/all/20250314163614.1356125-1-vincent.guittot@linaro.org/
> 
> The current Energy Aware Scheduler has some known limitations which have
> became more and more visible with features like uclamp as an example. This
> serie tries to fix some of those issues:
> - tasks stacked on the same CPU of a PD
> - tasks stuck on the wrong CPU.

I think you are under selling the importance of this improvement :-)

FWIW, in my view the new mechanism will help us:

1. Improve slow reaction time of lb. Waiting for another CPU to pull is very
   slow. And with 4ms TICK being the default and what I believe should be
   demolished (for most systems) back off mechanisms, when lb kicks in things
   has gone really bad already.
2. It helps implement misfit based on energy I brought up in the past [1]
3. It brings up a step closer to unify wake up and load balancer paths as we
   discussed is necessary to get a decent sched qos. We need to add the concept
   of task placement based on latency, and if lb can't take similar decision it
   is hard to make this useful. With push lb, both path can easily follow up
   the same decision tree.

I have backported an earlier version of this to help verify it, but so far
I think it is an amazing addition. Thanks for this!

[1] https://lore.kernel.org/lkml/20231209011759.398021-1-qyousef@layalina.io/

> 
> Patch 1 fixes the case where a CPU is wrongly classified as overloaded
> whereas it is capped to a lower compute capacity. This wrong classification
> can prevent periodic load balancer to select a group_misfit_task CPU
> because group_overloaded has higher priority.
> 
> Patch 2 removes the need of testing uclamp_min in cpu_overutilized to
> trigger the active migration of a task on another CPU.
> 
> Patch 3 prepares select_task_rq_fair() to be called without TTWU, Fork or
> Exec flags when we just want to look for a possible better CPU.
> 
> Patch 4 adds push call back mecanism to fair scheduler but doesn't enable
> it.
> 
> Patch 5 enable has_idle_core for !SMP system to track if there may be an
> idle CPU in the LLC.
> 
> Patch 6 adds some conditions to enable pushing runnable tasks for EAS:
> - when a task is stuck on a CPU and the system is not overutilized.
> - if there is a possible idle CPU when the system is overutilized.
> 
> More tests results will come later as I wanted to send the pachtset before
> LPC.
> 
> I have kept Tbench figures as I added them in v7 but results are the same
> with the correct patch 6.
> 
> Tbench on dragonboard rb5
> schedutil and EAS enabled
> 
> # process     tip                   +patchset
> 1              29.3(+/-0.3%)        29.2(+/-0.2%) +0%
> 2              61.1(+/-1.8%)        61.7(+/-3.2%) +1%
> 4             260.0(+/-1.7%)       258.8(+/-2.8%) -1%       
> 8            1361.2(+/-3.1%)      1377.1(+/-1.9%) +1%
> 16            981.5(+/-0.6%)       958.0(+/-1.7%) -2%
> 
> Hackbench didn't show any difference
> 
> Changes since v7:
> - Rebased on latest tip/sched/core
> - Fix some typos
> - Fix patch 6 mess 
> 
> Vincent Guittot (6):
>   sched/fair: Filter false overloaded_group case for EAS
>   sched/fair: Update overutilized detection
>   sched/fair: Prepare select_task_rq_fair() to be called for new cases
>   sched/fair: Add push task mechanism for fair
>   sched/fair: Enable idle core tracking for !SMT
>   sched/fair: Add EAS and idle cpu push trigger
> 
>  kernel/sched/fair.c     | 350 +++++++++++++++++++++++++++++++++++-----
>  kernel/sched/sched.h    |  46 ++++--
>  kernel/sched/topology.c |   2 +
>  3 files changed, 345 insertions(+), 53 deletions(-)
> 
> -- 
> 2.43.0
>