linux-kernel - Re: [PATCH 09/17] sched/core: Add support for nohz

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Message-ID: <08ff261e-7ac8-4a1d-bceb-b1bf792171ae@linux.ibm.com>
Date: Fri, 21 Nov 2025 10:10:16 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: K Prateek Nayak <kprateek.nayak@....com>, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, tglx@...utronix.de, yury.norov@...il.com,
        maddy@...ux.ibm.com, srikar@...ux.ibm.com, gregkh@...uxfoundation.org,
        pbonzini@...hat.com, seanjc@...gle.com, vschneid@...hat.com,
        iii@...ux.ibm.com, huschle@...ux.ibm.com, rostedt@...dmis.org,
        dietmar.eggemann@....com, christophe.leroy@...roup.eu
Subject: Re: [PATCH 09/17] sched/core: Add support for nohz_full CPUs

Hi Prateek.

On 11/21/25 8:46 AM, K Prateek Nayak wrote:
> Hello Shrikanth,
> 
> On 11/19/2025 6:14 PM, Shrikanth Hegde wrote:
>> Enable tick on nohz full CPU when it is marked as paravirt.
>> If there in no CFS/RT running there, disable the tick to save the power.
> 
> Wouldn't the task be pinned if it is running on a nohz CPU?
> We don't push out pinned tasks so this seems unnecessary.
> Am I missing something?
> 

No. Task will not be pinned if it running on nohz_full unless
user has done a taskset. They are still part of sched domains.

Pinning is usually true for isolcpus. You need to explicity set taskset
for isolcpus since by default you won't go to those CPUs.

on nohz_full it is just that tick will be disabled when it has
only one task running. If there are more task on it, tick will be
enabled and load balancing can run.

Example:  ( i have 300-479 as nohz_full)
taskset -c 300-315 stress-ng --cpu=2

(it was initially on 301,302)
10:27:37 PM  301  100.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00
10:27:37 PM  302  100.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00

Ran hackbench for a brief time. After hackbench completes,
(it runs now on 301,314)
10:27:43 PM  301  100.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00
10:27:43 PM  314  100.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00