Portable RK is a portable implementation of a resource kernel, a resource-centric approach to build a real-time kernel that provides explicit timely, guaranteed, and enforced access by applications to system resources. Portable RK is designed to work with widely available operating systems with minimal changes. This facilitates experimentation in familiar software environments and helps the faster deployment of research results. Execution in resource kernels is directly based on OS-enforced resource reservation. As a result, an application can request the reservation of a certain amount of a resource, and the kernel can guarantee that the requested amount is available to that application in timely fashion. In this paper, we describe the design and implementation of Portable RK called Linux/RK that resides within the Linux kernel. The evaluation results show that Portable RK in the form of Linux/RK gives direct control over timely resource utilization by applications and that its overhead costs are small enough to be negligible.