Hello, Michael. On Thu, Jul 27, 2017 at 03:15:47PM -0500, Michael Bringmann wrote: > There is an underlying assumption in many layers / modules of the Linux > system that CPU <-> node mapping is static. This is despite the presence > of features like NUMA and 'hotplug' that support the dynamic addition/ > removal of fundamental system resources like CPUs and memory. PowerPC > systems, however, do provide extensive features for the dynamic change > of resources available to a system.
The text can go as-is but keeping cpu <-> node mapping static is a trade-off rather than upper layers missing out something. Making cpu <-> node mapping dynamic means adding complexities and overhead to a lot hotter paths including memory allocation. It's just a better trade off to keep the mapping static from arch side even if that means we have to use more complex mapping in arch and/or allocate more possible cpus than strictly necessary. > Currently, there is little or no synchronization protection around the > updating of the CPU <-> node mapping, and the export/update of this > information for other layers / modules. In systems which can change > this mapping during 'hotplug', like PowerPC, the information is changing > underneath all layers that might reference it. > > This patch attempts to ensure that a valid, usable cpumask attribute is > used by the workqueue infrastructure when setting up new resource pools. > It prevents a crash that has been observed when an 'empty' cpumask is > passed along to the worker/task scheduling code. It is intended as an > intermediate fix until a more fundamental review and correction of the > issue can be done. Thanks a lot for your patience! Much appreciated. -- tejun
