linux-kernel - Re: [RFC PATCH v4 00/17] Paravirt CPUs and push task for less vCPU preemption

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Message-ID: <b609cb77-a64e-4743-b1b6-ec043f8174a0@linux.ibm.com>
Date: Wed, 19 Nov 2025 18:23:33 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        gregkh@...uxfoundation.org, christophe.leroy@...roup.eu
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, 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: [RFC PATCH v4 00/17] Paravirt CPUs and push task for less vCPU
 preemption



On 11/19/25 11:50 AM, Shrikanth Hegde wrote:
> Detailed problem statement and some of the implementation choices were
> discussed earlier[1].
> 
> [1]: https://lore.kernel.org/all/20250910174210.1969750-1-sshegde@linux.ibm.com/
> 
> This is likely the version which would be used for LPC2025 discussion on
> this topic. Feel free to provide your suggestion and hoping for a solution
> that works for different architectures and it's use cases.
> 
> All the existing alternatives such as cpu hotplug, creating isolated
> partitions etc break the user affinity. Since number of CPUs to use change
> depending on the steal time, it is not driven by User. Hence it would be
> wrong to break the affinity. This series allows if the task is pinned
> only paravirt CPUs, it will continue running there.
> 
> Changes compared v3[1]:
> 
> - 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 helper patch to override arch behaviour for easier debugging
>    during development.

Sorry for creating confusion around last two patches. Sent out new version.

https://lore.kernel.org/all/20251119124449.1149616-1-sshegde@linux.ibm.com/