linux-kernel - Re: [PATCH 0/3] sched: Generalize misfit load balance

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Message-ID: <179693ab-74d1-4358-8fdf-04cdcb80a814@arm.com>
Date: Thu, 21 Dec 2023 16:26:31 +0100
From: Pierre Gondois <pierre.gondois@....com>
To: Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>
Cc: linux-kernel@...r.kernel.org, Lukasz Luba <lukasz.luba@....com>,
 Wei Wang <wvw@...gle.com>, Rick Yiu <rickyiu@...gle.com>,
 Chung-Kai Mei <chungkai@...gle.com>
Subject: Re: [PATCH 0/3] sched: Generalize misfit load balance

Hello Qais,

On 12/9/23 02:17, Qais Yousef wrote:
> Misfit load balance was added to help handle HMP systems where we can make
> a wrong decision at wake up thinking a task can run at a smaller core, but its
> characteristics change and requires to migrate to a bigger core to meet its
> performance demands.
> 
> With the addition of uclamp, we can encounter more cases where such wrong
> placement decisions can be made and require load balancer to do a corrective
> action.
> 
> Specifically if a big task capped by uclamp_max was placed on a big core at
> wake up because EAS thought it is the most energy efficient core at the time,
> the dynamics of the system might change where other uncapped tasks might wake
> up on the cluster and there could be a better new more energy efficient
> placement for the capped task(s).
> 
> We can generalize the misfit load balance to handle different type of misfits
> (whatever they may be) by simply giving it a reason. The reason can decide the
> type of action required then.
> 
> Current misfit implementation is considered MISFIT_PERF. Which means we need to
> move a task to a better CPU to meet its performance requirement.
> 
> For UCLAMP_MAX I propose MISFIT_POWER, where we need to find a better placement
> to control its impact on power.
> 
> Once we have an API to annotate latency sensitive tasks, it is anticipated
> MISFIT_LATENCY load balance will be required to help handle oversubscribe
> situations to help better distribute the latency sensitive tasks to help reduce
> their wake up latency.
> 
> Patch 1 splits misfit status update from misfit detection by adding a new
> function is_misfit_task().
> 
> Patch 2 implements the generalization logic by adding a misfit reason and
> propagating that correctly and guarding the current misfit code with
> MISFIT_PERF reason.
> 
> Patch 3 is an RFC on a potential implementation for MISFIT_POWER.
> 
> Patch 1 and 2 were tested stand alone and had no regression observed and should
> not introduce a functional change and can be considered for merge if they make
> sense after addressing any review comments.
> 
> Patch 3 was only tested to verify it does what I expected it to do. But no real
> power/perf testing was done. Mainly because I was expecting to remove uclamp
> max-aggregation [1] and the RFC I currently have (which I wrote many many
> months ago) is tied to detecting a task being uncapped by max-aggregation.
> I need to rethink the detection mechanism.

I tried to trigger the MISFIT_POWER misfit reason without success so far.
Would it be possible to provide a workload/test to reliably trigger the
condition ?

Regards,
Pierre

> 
> Beside that, the logic relies on using find_energy_efficient_cpu() to find the
> best potential new placement for the task. To do that though, we need to force
> every CPU to do the MISFIT_POWER load balance as we don't know which CPU should
> do the pull. But there might be better thoughts on how to handle this. So
> feedback and thoughts would be appreciated.
> 
> [1] https://lore.kernel.org/lkml/20231208015242.385103-1-qyousef@layalina.io/
> 
> Thanks!
> 
> --
> Qais Yousef
> 
> Qais Yousef (3):
>    sched/fair: Add is_misfit_task() function
>    sched/fair: Generalize misfit lb by adding a misfit reason
>    sched/fair: Implement new type of misfit MISFIT_POWER
> 
>   kernel/sched/fair.c  | 115 +++++++++++++++++++++++++++++++++++++------
>   kernel/sched/sched.h |   9 ++++
>   2 files changed, 110 insertions(+), 14 deletions(-)
>