Baby-CPU-Scheduler

This is a very basic and lightweight yet very performant CPU scheduler. You can use it for learning purposes as a base ground CPU scheduler on Linux. Notice that many features are disabled to make this scheduler as simple as possible.

Baby Scheduler is very lightweight and powerful for normal usage. I am using (baby-dl) as my main scheduler and sometimes I switch back to CacULE for testing. The throughput in Baby Scheduler is higher due to the task loadbalancer that I made for Baby Scheduler. The loadbalancing is done via only one CPU which is CPU0 in which CPU0 scan all other CPUs and move one task in every tick. The balancing is only depending on the number of tasks (i.e. no load, weight or other factors).

Baby scheduler is only 1036 LOC where 254 LOC of it are just dependent functions that are copied from CFS without any changes to let the scheduler compile and run. You can find all Baby code in bs.c, bs.h, and numa_fair.h.

Flavors

Currently there are three flavors of Baby Scheduler

• Deadline Scheduling (dl) - main
• Virtual Run Time Scheduling (vrt)
• Round Robin Scheduling (rr)

All the three flavors have the same tasks load balancing method. They only differ in the strategy of picking the next task to run, and other minor differences.

Deadline Scheduling

Baby's main scheduling is the deadline flavor. The scheduler picks the task with the earliest deadline. A new task gets a deadline = now() + 1.180ms. The task with earliest deadline will be picked and run on the CPU until it gets to sleep or another task had earlier deadline. While the task is running, its deadline gets updated only when its deadline is past - compared with the current time (now). So, basically we don't want to update its deadline with now() + 1.180ms on every tick, otherwise, I call this situation by horse and carrot. I am using the deadline as a slice too, we don't want to keep preempting tasks in which their deadlines are very close to each other. The best solution is to give a minimum/maximum slice to the running task to at least gets its next updated deadline to be not very close to the competing task. This can save some context switching. Anyway, our deadline/slice is not too hight, it is only a 1.180ms.

Virtual Run Time Scheduling

The scheduler picks next task that has least vruntime, however, all CFS load/weight for task priority are replaced with a simpler mechanism. Tasks priorities are injected in vruntime where NICE0 priority task has a vruntime = real_exec_time, but NICE-20 task has a vruntime < real_exec_time in which NICE-20 task will run 20 more milliseconds than NICE0 one in a race time of 40ms. See the equation in kernel/sched/bs.c:convert_to_vruntime.

Round Robin Scheduling

Patch and Compile

Patch

First, you need to patch the kernel source with one of the flavors of baby scheduler. See an example of patching linux kernel here. You can use the same method to patch with baby instead of cacule.

1. Download the linux kernel (https://www.kernel.org/) that is same version as the patch (i.e if patch file name is baby-5.14.patch, then download https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.14.9.tar.xz)
2. Unzip linux kernel
3. Download baby patch file and place it inside the just unzipped linux kernel folder
4. cd linux-(version)
5. patch -p1 < baby-5.14.patch

Configure

Baby uses periodic HZ only, and you need to disable other scheduler features such as CGROUP_SCHED and stat/debuging features.

run make menuconfig and choose CONFIG_HZ_PERIODIC

You should see this when you run cat .config | grep -i hz:

CONFIG_HZ_PERIODIC=y
# CONFIG_NO_HZ_IDLE is not set
# CONFIG_NO_HZ_FULL is not set
# CONFIG_NO_HZ is not set

Then disable the following:

• CONFIG_EXPERT
• CONFIG_DEBUG_KERNEL
• CONFIG_SCHED_DEBUG
• CONFIG_SCHEDSTATS
• NO_HZ
• SCHED_AUTOGROUP
• CGROUP_SCHED
• UCLAMP_TASK
• SCHED_CORE

Make sure that CONFIG_BS_SCHED is selected (it appears at the top when running make menuconfig). Confirm by running cat .config | grep -i bs_sched

CONFIG_BS_SCHED=y

Now compile make
Then install the modules sudo make modules_install
Then install the kernel sudo make install
Reboot and choose the new kernel

To confirm that Baby is currently running:

$ dmesg | grep "Baby CPU"
Baby CPU scheduler (dl) v5.14 by Hamad Al Marri.