Re: [RFC v2 1/9] sched/docs: Document avoid_cpu_mask and avoid CPU concept

[Hillf Danton <hdanton@sina.com>]

On Thu, 26 Jun 2025 00:41:00 +0530 Shrikanth Hegde wrote
> This describes what avoid CPU means and what scheduler aims to do 
> when a CPU is marked as avoid. 
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
>  Documentation/scheduler/sched-arch.rst | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/Documentation/scheduler/sched-arch.rst b/Documentation/scheduler/sched-arch.rst
> index ed07efea7d02..d32755298fca 100644
> --- a/Documentation/scheduler/sched-arch.rst
> +++ b/Documentation/scheduler/sched-arch.rst
> @@ -62,6 +62,31 @@ Your cpu_idle routines need to obey the following rules:
>  arch/x86/kernel/process.c has examples of both polling and
>  sleeping idle functions.
>  
> +CPU Avoid
> +=========
> +
> +Under paravirt conditions it is possible to overcommit CPU resources.
> +i.e sum of virtual CPU(vCPU) of all VM is greater than number of physical
> +CPUs(pCPU). Under such conditions when all or many VM have high utilization,
> +hypervisor won't be able to satisfy the requirement and has to context switch
> +within or across VM. VM level context switch is more expensive compared to
> +task context switch within the VM.
> +
Sounds like VMs not well configured (or pCPUs not well partationed).

> +In such cases it is better that VM's co-ordinate among themselves and ask for
> +less CPU request by not using some of the vCPUs. Such vCPUs where workload
> +can be avoided at the moment are called as "Avoid CPUs". Note that when the
> +pCPU contention goes away, these vCPUs can be used again by the workload.
> +
In the car cockpit scenario for example with type1 hypervisor, there is app
kicking watchdog bound to every vCPU, so no vCPU should be avoided.

> +Arch need to set/unset the vCPU as avoid in cpu_avoid_mask. When set, avoid
> +the CPU and when unset, use it as usual.
> +
> +Scheduler will try to avoid those CPUs as much as it can.
> +This is achived by
> +1. Not selecting those CPU at wakeup.
> +2. Push the task away from avoid CPU at tick.
> +3. Not selecting avoid CPU at load balance.
> +
> +This works only for SCHED_RT and SCHED_NORMAL.
>  
Sounds like forcing a pill down through Peter's throat because Steve's headache.