linux-kernel - Re: [PATCH 00/17] Paravirt CPUs and push task for less vCPU preemption

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Message-ID: <236f4925-dd3c-41ef-be04-47708c9ce129@linux.ibm.com>
Date: Tue, 25 Nov 2025 14:18:32 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: "Christophe Leroy (CS GROUP)" <chleroy@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>
Cc: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        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, pbonzini@...hat.com,
        seanjc@...gle.com, kprateek.nayak@....com, vschneid@...hat.com,
        iii@...ux.ibm.com, huschle@...ux.ibm.com, rostedt@...dmis.org,
        dietmar.eggemann@....com
Subject: Re: [PATCH 00/17] Paravirt CPUs and push task for less vCPU
 preemption

Hi Christophe, Greg

>>>
>>> There is no "v" for this series :(
>>>
>>
>> I thought about adding v1.
>>
>> I made it as PATCH from RFC PATCH since functionally it should
>> be complete now with arch bits. Since it is v1, I remember usually 
>> people send out without adding v1. after v1 had tags such as v2.
>>
>> I will keep v2 for the next series.
>>
> 
> But you are listing changes compared to v3, how can it be a v1 ? 
> Shouldn't it be a v4 ? Or in reality a v5 as you already sent a v4 here 
> [1].
> 
> [1] https://lore.kernel.org/all/20251119062100.1112520-1- 
> sshegde@...ux.ibm.com/
> 
> Christophe

Sorry about the confusion in numbers. Hopefully below helps for reviewing.
If there are no objections, I will keep next one as v2. Please let me know.

Revision logs:
++++++++++++++++++++++++++++++++++++++
RFC PATCH v4 -> PATCH (This series)
++++++++++++++++++++++++++++++++++++++
- Last two patches were sent out separate instead of being with series.
   Sent it as part of series.
- Use DEVICE_ATTR_RW instead (greg)
- Made it as PATCH since arch specific handling completes the
   functionality.

+++++++++++++++++++++++++++++++++
RFC PATCH v3 -> RFC PATCH v4
+++++++++++++++++++++++++++++++++
- Introduced computation of steal time in powerpc code.
- Derive number of CPUs to use and mark the remaining as paravirt based
   on steal values.
- Provide debugfs knobs to alter how steal time values being used.
- Removed static key check for paravirt CPUs (Yury)
- Removed preempt_disable/enable while calling stopper (Prateek)
- Made select_idle_sibling and friends aware of paravirt CPUs.
- Removed 3 unused schedstat fields and introduced 2 related to paravirt
   handling.
- Handled nohz_full case by enabling tick on it when there is CFS/RT on
   it.
- Updated debug patch to override arch behavior for easier debugging
   during development.
- Kept the method to push only current task out instead of moving all task's
   on rq given the complexity of later.

+++++++++++++++++++++++++++++++++
RFC v2 -> RFC PATCH v3
+++++++++++++++++++++++++++++++++
- Renamed to paravirt_cpus_mask
- Folded the changes under CONFIG_PARAVIRT.
- Fixed the crash due work_buf corruption while using
   stop_one_cpu_nowait.
- Added sysfs documentation.
- Copy most of __balance_push_cpu_stop to new one, this helps it move
   the code out of CONFIG_HOTPLUG_CPU.
- Some of the code movement suggested.

+++++++++++++++++++++++++++++++++
RFC PATCH -> RFC v2
+++++++++++++++++++++++++++++++++
- Renamed to cpu_avoid_mask in place of cpu_parked_mask.
- Used a static key such that no impact to regular case.
- add sysfs file to show avoid CPUs.
- Make RT understand avoid CPUs.
- Add documentation patch
- Took care of reported compile error when NR_CPUS=1


PATCH          : https://lore.kernel.org/all/20251119124449.1149616-1-sshegde@linux.ibm.com/
RFC PATCH v4   : https://lore.kernel.org/all/20251119062100.1112520-1-sshegde@linux.ibm.com/#r
RFC PATCH v3   : https://lore.kernel.org/all/20250910174210.1969750-1-sshegde@linux.ibm.com/#r
RFC v2         : https://lore.kernel.org/all/20250625191108.1646208-1-sshegde@linux.ibm.com/#r
RFC PATCH      : https://lore.kernel.org/all/20250523181448.3777233-1-sshegde@linux.ibm.com/