On 1/4/26 2:09 AM, Chen Ridong wrote:
On 2026/1/2 3:15, Waiman Long wrote:Commit fe8cd2736e75 ("cgroup/cpuset: Delay setting of CS_CPU_EXCLUSIVE until valid partition") introduced a new check to disallow the setting of a new cpuset.cpus.exclusive value that is a superset of a sibling's cpuset.cpus value so that there will at least be one CPU left in the sibling in case the cpuset becomes a valid partition root. This new check does have the side effect of failing a cpuset.cpus change that make it a subset of a sibling's cpuset.cpus.exclusive value. With v2, users are supposed to be allowed to set whatever value they want in cpuset.cpus without failure. To maintain this rule, the check is now restricted to only when cpuset.cpus.exclusive is being changed not when cpuset.cpus is changed.Hi, Longman, You've emphasized that modifying cpuset.cpus should never fail. While I haven't found this explicitly documented. Should we add it? More importantly, does this mean the "never fail" rule has higher priority than the exclusive CPU constraints? This seems to be the underlying assumption in this patch.
Before the introduction of cpuset partition, writing to cpuset.cpus will only fail if the cpu list is invalid like containing CPUs outside of the valid cpu range. What I mean by "never-fail" is that if the cpu list is valid, the write action should not fail. The rule is not explicitly stated in the documentation, but it is a pre-existing behavior which we should try to keep to avoid breaking existing applications.
The exclusive CPU constraint does not apply to cpuset.cpus. It only applies when setting cpuset.cpus.exclusive wrt to other cpuset.cpus.exclusive* in sibling cpusets. So I will not say one has higher priority than the other.
Cheers, Longman
