Language Selection

English French German Italian Portuguese Spanish

Linux 5.3, LWN's Kernel Coverage and the Linux Foundation

Filed under
Linux
  • Linux 5.3 Enables "-Wimplicit-fallthrough" Compiler Flag

    The recent work on enabling "-Wimplicit-fallthrough" behavior for the Linux kernel has culminated in Linux 5.3 with actually being able to universally enable this compiler feature.

    The -Wimplicit-fallthrough flag on GCC7 and newer warns of cases where switch case fall-through behavior could lead to potential bugs / unexpected behavior.

  • EXT4 For Linux 5.3 Gets Fixes & Faster Case-Insensitive Lookups

    The EXT4 file-system updates have already landed for the Linux 5.3 kernel merge window that opened this week.

    For Linux 5.3, EXT4 maintainer Ted Ts'o sent in primarily a hearty serving of fixes. There are fixes from coverity warnings being addressed to typos and other items for this mature and widely-used Linux file-system.

  • Providing wider access to bpf()

    The bpf() system call allows user space to load a BPF program into the kernel for execution, manipulate BPF maps, and carry out a number of other BPF-related functions. BPF programs are verified and sandboxed, but they are still running in a privileged context and, depending on the type of program loaded, are capable of creating various types of mayhem. As a result, most BPF operations, including the loading of almost all types of BPF program, are restricted to processes with the CAP_SYS_ADMIN capability — those running as root, as a general rule. BPF programs are useful in many contexts, though, so there has long been interest in making access to bpf() more widely available. One step in that direction has been posted by Song Liu; it works by adding a novel security-policy mechanism to the kernel.
    This approach is easy enough to describe. A new special device, /dev/bpf is added, with the core idea that any process that has the permission to open this file will be allowed "to access most of sys_bpf() features" — though what comprises "most" is never really spelled out. A non-root process that wants to perform a BPF operation, such as creating a map or loading a program, will start by opening this file. It then must perform an ioctl() call (BPF_DEV_IOCTL_GET_PERM) to actually enable its ability to call bpf(). That ability can be turned off again with the BPF_DEV_IOCTL_PUT_PERM ioctl() command.

    Internally to the kernel, this mechanism works by adding a new field (bpf_flags) to the task_struct structure. When BPF access is enabled, a bit is set in that field. If this patch goes forward, that detail is likely to change since, as Daniel Borkmann pointed out, adding an unsigned long to that structure for a single bit of information is unlikely to be popular; some other location for that bit will be found.

  • The io.weight I/O-bandwidth controller

    Part of the kernel's job is to arbitrate access to the available hardware resources and ensure that every process gets its fair share, with "its fair share" being defined by policies specified by the administrator. One resource that must be managed this way is I/O bandwidth to storage devices; if due care is not taken, an I/O-hungry process can easily saturate a device, starving out others. The kernel has had a few I/O-bandwidth controllers over the years, but the results have never been entirely satisfactory. But there is a new controller on the block that might just get the job done.
    There are a number of challenges facing an I/O-bandwidth controller. Some processes may need a guarantee that they will get at least a minimum amount of the available bandwidth to a given device. More commonly in recent times, though, the focus has shifted to latency: a process should be able to count on completing an I/O request within a bounded period of time. The controller should be able to provide those guarantees while still driving the underlying device at something close to its maximum rate. And, of course, hardware varies widely, so the controller must be able to adapt its operation to each specific device.

    The earliest I/O-bandwidth controller allows the administrator to set maximum bandwidth limits for each control group. That controller, though, will throttle I/O even if the device is otherwise idle, causing the loss of I/O bandwidth. The more recent io.latency controller is focused on I/O latency, but as Tejun Heo, the author of the new controller, notes in the patch series, this controller really only protects the lowest-latency group, penalizing all others if need be to meet that group's requirements. He set out to create a mechanism that would allow more control over how I/O bandwidth is allocated to groups.

  • TurboSched: the return of small-task packing

    CPU scheduling is a difficult task in the best of times; it is not trivial to pick the next process to run while maintaining fairness, minimizing energy use, and using the available CPUs to their fullest potential. The advent of increasingly complex system architectures is not making things easier; scheduling on asymmetric systems (such as the big.LITTLE architecture) is a case in point. The "turbo" mode provided by some recent processors is another. The TurboSched patch set from Parth Shah is an attempt to improve the scheduler's ability to get the best performance from such processors.
    Those of us who have been in this field for far too long will, when seeing "turbo mode", think back to the "turbo button" that appeared on personal computers in the 1980s. Pushing it would clock the processor beyond its original breathtaking 4.77MHz rate to something even faster — a rate that certain applications were unprepared for, which is why the "go slower" mode was provided at all. Modern turbo mode is a different thing, though, and it's not just a matter of a missing front-panel button. In short, it allows a processor to be overclocked above its rated maximum frequency for a period of time when the load on the rest of system overall allows it.

    Turbo mode can thus increase the CPU cycles available to a given process, but there is a reason why the CPU's rated maximum frequency is lower than what turbo mode provides. The high-speed mode can only be sustained as long as the CPU temperature does not get too high and, crucially (for the scheduler), the overall power load on the system must not be too high. That, in turn, implies that some CPUs must be powered down; if all CPUs are running, there will not be enough power available for any of those CPUs to go into the turbo mode. This mode, thus, is only usable for certain types of workloads and will not be usable (or beneficial) for many others.

  • EdgeX Foundry Announces Production Ready Release Providing Open Platform for IoT Edge Computing to a Growing Global Ecosystem

    EdgeX Foundry, a project under the LF Edge umbrella organization within the Linux Foundation that aims to establish an open, interoperable framework for edge IoT computing independent of hardware, silicon, application cloud, or operating system, today announced the availability of its “Edinburgh” release. Created collaboratively by a global ecosystem, EdgeX Foundry’s new release is a key enabler of digital transformation for IoT use cases and is a platform for real-world applications both for developers and end users across many vertical markets. EdgeX community members have created a range of complementary products and services, including commercial support, training and customer pilot programs and plug-in enhancements for device connectivity, applications, data and system management and security.

    Launched in April 2017, and now part of the LF Edge umbrella, EdgeX Foundry is an open source, loosely-coupled microservices framework that provides the choice to plug and play from a growing ecosystem of available third party offerings or to augment proprietary innovations. With a focus on the IoT Edge, EdgeX simplifies the process to design, develop and deploy solutions across industrial, enterprise, and consumer applications.