We present ChronOS Linux, a best-effort real-time Linux kernel for chip multiprocessors (CMPs). ChronOS addresses the intersection of three problem spaces: a) OS-support for obtaining best-effort timing assurances, b) real-time Linux kernel augmented with the PREEMPT_RT patch, and c) OS support for CMP-aware real-time scheduling. While each of these spaces have been studied in the past, their intersection, which has strong problem motivations, was previously empty. Best-effort timeliness targets real-time applications with run-time uncertainties and resource overloads, and optimizes collective application timeliness — as specified by the application. ChronOS directly supports the implementation of best-effort real-time schedulers on CMPs, in addition to others, in the global and partitioned scheduling disciplines. ChronOS extends the PREEMPT RT Linux patch, and thus provides full kernel preemptibility and retains stock Linux features. We validate our claims by reporting on the ...