3.14. Offloading RCU Callbacks
The Read-Copy-Update (RCU) system is a lockless mechanism for mutual exclusion inside the kernel. As a consequence of performing RCU operations, call-backs are sometimes queued on CPUs to be performed at a future moment when removing memory is safe.
RCU callbacks can be offloaded using the
rcu_nocbs and rcu_nocb_poll kernel parameters.
- To remove one or more CPUs from the candidates for running RCU callbacks, specify the list of CPUs in the
rcu_nocbskernel parameter, for example:rcu_nocbs=1,4-6
orrcu_nocbs=3
The second example instructs the kernel that CPU 3 is a no-callback CPU. This means that RCU callbacks will not be done in thercuc/$CPUthread pinned to CPU 3, but in thercuo/$CPUthread, which can be moved to a housekeeping CPU, relieving CPU 3 from doing RCU callbacks job.To move RCU callback threads to the housekeeping CPU, use thetuna -t rcu* -c X -mcommand, where X denotes the housekeeping CPU. For example, in a system where CPU 0 is the housekeeping CPU, all RCU callback threads can be moved to CPU 0 using this command:~]#
tuna -t rcu* -c 0 -mThis relieves all CPUs other than CPU 0 from doing RCU work. - Although the RCU offload threads can perform the RCU callbacks on another CPU, each CPU is responsible for awakening the corresponding RCU offload thread. To relieve each CPU from the responsibility of awakening their RCU offload threads, set the
rcu_nocb_pollkernel parameter:rcu_nocb_poll
Withrcu_nocb_pollset, the RCU offload threads will be periodically raised by a timer to check if there are callbacks to run.
A common use case for these two options is:
- Using
rcu_nocbs=cpulistto allow the user to move all RCU offload threads to a housekeeping CPU; - Setting
rcu_nocb_pollto relieve each CPU from the responsibility awakening their RCU offload threads.
This combination reduces the interference on CPUs that are specialized for the user's workload.
For more information about RCU tuning on real-time, see Avoiding RCU Stalls in the real-time kernel.
