The CPU bandwidth quota and burst controls currently make the order of userspace writes significant. A burst configured while quota is unlimited can prevent a later finite quota from being installed, and updating the burst before increasing the quota can fail with -EINVAL.
Remove the write-order dependency by decoupling burst validation from the current quota. The previous quota-relative checks implicitly limited the maximum configurable burst to max_bw_runtime_us / 2 for a finite quota, so retain that limit as an independent upper bound. Keep the configured burst unchanged across quota updates, and cap the burst actually available at refill time to min(burst, quota). This allows cpu.max and cpu.max.burst to be updated in either order without changing the maximum burst previously allowed for a finite quota. The series also adds a selftest covering both write orders and documents the resulting behavior for cgroup v1 and cgroup v2. --- Changes in v3: - Use max_bw_runtime_us / 2 as an independent upper bound for burst, preserving the maximum burst previously allowed with a finite quota while removing its dependency on the current quota. - Drop Cc: [email protected] from patch 1. - Update the cpu.max.burst documentation to describe the configured burst independently of the current quota and the quota-relative clamp applied at runtime refill. Changes in v2: - Keep the configured burst independent of quota changes instead of adjusting or resetting it when the quota is updated. - Move the quota-relative restriction to the CFS runtime refill path, where the effective burst is capped to the current quota. - Add selftests covering quota and burst updates in both write orders and document the resulting behavior for cgroup v1 and cgroup v2. v2: https://lore.kernel.org/all/[email protected]/ v1: https://lore.kernel.org/all/[email protected]/ Zhe Liu (3): sched/fair: remove quota/burst write-order dependency selftests: cgroup: test CPU quota and burst write order docs: cgroup-v2: document CPU quota and burst ordering Documentation/admin-guide/cgroup-v2.rst | 7 +++- Documentation/scheduler/sched-bwc.rst | 22 ++++++++----- kernel/sched/core.c | 3 +- kernel/sched/fair.c | 3 +- tools/testing/selftests/cgroup/test_cpu.c | 40 +++++++++++++++++++++++ 5 files changed, 62 insertions(+), 13 deletions(-) base-commit: df2908090cda368b01ff43709f51890076c56157 -- 2.25.1
