Housekeeping

CPU Isolation moves away kernel work that may otherwise run on any CPU. The purpose of its related features is to reduce the OS jitter that some extreme workloads can’t stand, such as in some DPDK usecases.

The kernel work moved away by CPU isolation is commonly described as “housekeeping” because it includes ground work that performs cleanups, statistics maintainance and actions relying on them, memory release, various deferrals etc...

Sometimes housekeeping is just some unbound work (unbound workqueues, unbound timers, ...) that gets easily assigned to non-isolated CPUs. But sometimes housekeeping is tied to a specific CPU and requires elaborated tricks to be offloaded to non-isolated CPUs (RCU_NOCB, remote scheduler tick, etc...).

Thus, a housekeeping CPU can be considered as the reverse of an isolated CPU. It is simply a CPU that can execute housekeeping work. There must always be at least one online housekeeping CPU at any time. The CPUs that are not isolated are automatically assigned as housekeeping.

Housekeeping is currently divided in four features described by the enum hk_type type:

  1. HK_TYPE_DOMAIN matches the work moved away by scheduler domain isolation performed through isolcpus=domain boot parameter or isolated cpuset partitions in cgroup v2. This includes scheduler load balancing, unbound workqueues and timers.

  2. HK_TYPE_KERNEL_NOISE matches the work moved away by tick isolation performed through nohz_full= or isolcpus=nohz boot parameters. This includes remote scheduler tick, vmstat and lockup watchdog.

  3. HK_TYPE_MANAGED_IRQ matches the IRQ handlers moved away by managed IRQ isolation performed through isolcpus=managed_irq.

  4. HK_TYPE_DOMAIN_BOOT matches the work moved away by scheduler domain isolation performed through isolcpus=domain only. It is similar to HK_TYPE_DOMAIN except it ignores the isolation performed by cpusets.

Housekeeping cpumasks

Housekeeping cpumasks include the CPUs that can execute the work moved away by the matching isolation feature. These cpumasks are returned by the following function:

const struct cpumask *housekeeping_cpumask(enum hk_type type)

By default, if neither nohz_full=, nor isolcpus, nor cpuset’s isolated partitions are used, which covers most usecases, this function returns the cpu_possible_mask.

Otherwise the function returns the cpumask complement of the isolation feature. For example:

With isolcpus=domain,7 the following will return a mask with all possible CPUs except 7:

housekeeping_cpumask(HK_TYPE_DOMAIN)

Similarly with nohz_full=5,6 the following will return a mask with all possible CPUs except 5,6:

housekeeping_cpumask(HK_TYPE_KERNEL_NOISE)

Synchronization against cpusets

Cpuset can modify the HK_TYPE_DOMAIN housekeeping cpumask while creating, modifying or deleting an isolated partition.

The users of HK_TYPE_DOMAIN cpumask must then make sure to synchronize properly against cpuset in order to make sure that:

  1. The cpumask snapshot stays coherent.

  2. No housekeeping work is queued on a newly made isolated CPU.

  3. Pending housekeeping work that was queued to a non isolated CPU which just turned isolated through cpuset must be flushed before the related created/modified isolated partition is made available to userspace.

This synchronization is maintained by an RCU based scheme. The cpuset update side waits for an RCU grace period after updating the HK_TYPE_DOMAIN cpumask and before flushing pending works. On the read side, care must be taken to gather the housekeeping target election and the work enqueue within the same RCU read side critical section.

A typical layout example would look like this on the update side (housekeeping_update()):

rcu_assign_pointer(housekeeping_cpumasks[type], trial);
synchronize_rcu();
flush_workqueue(example_workqueue);

And then on the read side:

rcu_read_lock();
cpu = housekeeping_any_cpu(HK_TYPE_DOMAIN);
queue_work_on(cpu, example_workqueue, work);
rcu_read_unlock();