Hello, Michael. On Thu, Jul 27, 2017 at 02:07:53PM -0500, Michael Bringmann wrote: > The problem lies with the ordering of events with respect to the order in > which we add (or remove) CPUs to NUMA systems, and make use of that knowledge.
Isn't the root cause that the upper layers including workqueue expect cpu <-> node mapping to be static but powerpc doesn't follow that? I don't get why ordering matters here. > The CPUs present are assigned to nodes, and workqueues and their > infrastructure > are created to use the CPUs in a node. Workqueues are created at boot time > and updated or created as CPUs are added or removed. However, there is little > or no synchronization or ordering of these events, and the data structures What I meant was that there's no synchronization construct protection cpu <-> node mapping. If arch code changes it during hot plug, it's changing it underneath anybody who might be using that association. > mapping CPUs to nodes may not be updated before the workqueue infrastructure > is built for a node. Thus we have the possibility of an invalid CPU mask > attribute being attached to a newly created workqueue before the CPUs have > been properly registered and published to a node. > > This patch attempts to provide a partial ordering of events within workqueue > by delaying the use of newly calculated CPU masks as the value for a workqueue > attribute until they have valid content. Instead the workqueue code must > delay > creating new workqueues until this function succeeds, or it can use a > previously > calculated cpumask attribute that is known to be valid. > > This patch attempts to ensure that a valid, usable cpumask is used to set up > newly created pools for workqueues. This patch provides a fix for NUMA > systems > which can add/subtract processors dynamically. The patch is expected to be an > intermediate one while developers find and correct any underlying issues. And what this patch does is adding a bandaid so that we at least don't crash immediately when this condition triggers until the arch code can be fixed properly. Thanks. -- tejun
