3.14. Offloading RCU Callbacks

To remove one or more CPUs from the candidates for running RCU callbacks, specify the list of CPUs in the rcu_nocbs kernel parameter, for example:

rcu_nocbs=1,4-6

rcu_nocbs=1,4-6

or

rcu_nocbs=3

rcu_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 the rcuc/$CPU thread pinned to CPU 3, but in the rcuo/$CPU thread, 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 the tuna -t rcu* -c X -m command, 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 -m

~]# tuna -t rcu* -c 0 -m

This 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_poll kernel parameter:

rcu_nocb_poll

rcu_nocb_poll

With rcu_nocb_poll set, the RCU offload threads will be periodically raised by a timer to check if there are callbacks to run.