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Workqueue is asynchronous execution mechanism in kernel used to defer function executions to process context and used for various purposes in the kernel - to decouple execution of time-consuming tasks, to handle tasks which require process-context from places which can't sleep and so on.
Like most in-kernel infrastructure, workqueue has been constantly evolving as new usages and requirements arise and we learn new and better ways to implement it.
This presentation gives overview of workqueue and follows through its evolution in the hope of showing how design decisions accumulate and affect long-term trajectory of development, the dynamics between new features and existing behaviors, and how compatibility often becomes the over-ruling criterion when making design decisions.