Enforcing safety of real-time schedules on contemporary processors using a virtual simple architecture (VISA)

Determining safe and tight upper bounds on the worst-case execution time (WCET) of hard real-time tasks running on contemporary microarchitectures is a difficult problem. Current trends in microarchitecture design have created a complexity wall: by enhancing performance through ever more complex architectural features, systems have become increasingly hard to analyze. This paper extends a framework, introduced previously as virtual simple architecture (VISA), to multitasking real-time systems. The objective of VISA is to obviate the need to statically analyze complex processors by instead shifting the burden of guaranteeing deadlines - in part - onto the hardware. The VISA framework exploits a complex processor that ordinarily operates with all of its advanced features enabled, called the complex mode, but which can also be downgraded to a simple mode by gating off the advanced features. A WCET bound is statically derived for a task assuming the simple mode. However, this abstraction is speculatively undermined at run-time by executing the task in the complex mode. The task's progress is continuously gauged to detect anomalous cases in which the complex mode underperforms, in which case the processor switches to the simple mode to explicitly enforce the overall contractual WCET. The processor typically operates in complex mode, generating significant slack, and the VISA safety mechanism ensures bounded timing in atypical cases. Extra slack can be exploited for reducing power consumption and/or enhancing functionality. By extending VISA from single-task to multi-tasking systems, this paper reveals the full extent of VISA'S powerful abstraction capability. Key missing pieces are filled in: (1) preserving integrity of the gauging mechanism despite disruptions caused by preemptions; (2) demonstrating compatibility with arbitrary scheduling and dynamic voltage scaling (DVS) policies; (3) formally describing VISA speculation overheads in terms of padding tasks' WCETs; and (4) developing a systematic method for minimizing these overheads. We also propose a VISA variant that dynamically accrues the slack needed to facilitate speculation in the complex mode, eliminating the need to statically pad WCETs and thereby enabling VISA-style speculation even in highly-utilized systems.