This describes what avoid CPU means and what scheduler aims to do when a CPU is marked as avoid.
Signed-off-by: Shrikanth Hegde <sshe...@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. + +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. + +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. Possible arch/ problems ======================= -- 2.43.0
