RCU expedited mode

Description

You can use RCU expedited mode to reduce latencies when rcu_barrier() is called during startup.

Normally, RCU waits until multiple rcu_barriers have accumulated before servicing them. This is done to reduce the runtime overhead of processing this operations while a system is running. However, in expedited mode, the kernel will process the held rcu operations immediately. This incurs possible more total overhead for these RCU operations, but makes the system run faster. This is appropriate when trying to boot quickly.

After expedited mode is used during boot, by setting some options on the kernel command line, RCU is then put back into normal mode for regular runtime operation of the system.

How to implement or use

Add the following items to the kernel command line:

 rcupdate.rcu_normal_after_boot=0 rcupdate.rcu_expedited=1

Once the system is fully booted, execute the following:

 echo 1 > /sys/kernel/rcu_normal

This can be done by adding a unit file to systemd, or adding an init script in a "SYSV init"-based system. Make sure that the unit file or script is executed after the critical phase of booting is finished.

Expected Improvement

Need to fill in average savings from using this technique

Resources

Examples

RedHat uses this in an automotive distribution of Linux, here: https://gitlab.com/CentOS/automotive/src/automotive-image-builder/-/commit/40ecacca333eb75d8e40fb15ed7a544be18ade43

Here is the systemd unit file that they used to reset the system to rcu_normal mode:

[Unit]
Description=Disable expedited rcu updates
Requires=boot-complete.target
After=boot-complete.target

[Service]
Type=oneshot
ExecStart=/usr/bin/echo 1
StandardOutput=file:/sys/kernel/rcu_normal

[Install]
WantedBy=default.target

Projects

None

Specifications

I'm not sure if this is related to RCU Expedited Grace Periods or not.

Documentation

See https://www.kernel.org/doc/html/v4.14/admin-guide/kernel-parameters.html (search for rcupdate)

and https://lwn.net/Articles/777214/ for descriptions of rcupdate.rcu_expedited and rcupdate.rcu_normal_after_boot

The technique is mentioned in this presentation: https://pretalx.com/media/devconf-us-2024/submissions/HXVPHC/resources/Accelerating_Linux_Boot_Time__Techniques_an_jjA5HNE.pdf

Patches

None

Case Studies

Case 1

Hardware	hardware description
Kernel Version	put kernel version here
Configuration	list relevant kernel config options here
Time without using technique	xxxxxxx usec
Time using the technique	xxxxxxx usec
Time savings	mmm milliseconds
Submitter	name and company of submitter