CFS bandwidth currently distributes new runtime and unthrottles cfs_rq's inline in an hrtimer callback. Runtime distribution is a per-cpu operation, and unthrottling is a per-cgroup operation, since a tg walk is required. On machines with a large number of cpus and large cgroup hierarchies, this cpus*cgroups work can be too much to do in a single hrtimer callback: since IRQ are disabled, hard lockups may easily occur. Specifically, we've found this scalability issue on configurations with 256 cpus, O(1000) cgroups in the hierarchy being throttled, and high memory bandwidth usage. To fix this, we can instead unthrottle cfs_rq's asynchronously via a CSD. Each cpu is responsible for unthrottling itself, thus sharding the total work more fairly across the system, and avoiding hard lockups. Signed-off-by: Josh Don <joshdon@google.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lkml.kernel.org/r/20221117005418.3499691-1-joshdon@google.com