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Kernel: LWN Linux Articles Now Outside the Paywall

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Linux
  • What's a CPU to do when it has nothing to do?

    It would be reasonable to expect doing nothing to be an easy, simple task for a kernel, but it isn't. At Kernel Recipes 2018, Rafael Wysocki discussed what CPUs do when they don't have anything to do, how the kernel handles this, problems inherent in the current strategy, and how his recent rework of the kernel's idle loop has improved power consumption on systems that aren't doing anything.

    The idle loop, one of the kernel subsystems that Wysocki maintains, controls what a CPU does when it has no processes to run. Precise to a fault, Wysocki defined his terms: for the purposes of this discussion, a CPU is an entity that can take instructions from memory and execute them at the same time as any other entities in the same system are doing likewise. On a simple, single-core single-processor system, that core is the CPU. If the processor has multiple cores, each of those cores is a CPU. If each of those cores exposes multiple interfaces for simultaneous instruction execution, which Intel calls "hyperthreading", then each of those threads is a CPU.

  • New AT_ flags for restricting pathname lookup

    System calls like openat() have access to the entire filesystem — or, at least, that part of the filesystem that exists in the current mount namespace and which the caller has the permission to access. There are times, though, when it is desirable to reduce that access, usually for reasons of security; that has proved to be especially true in many container use cases. A new patch set from Aleksa Sarai has revived an old idea: provide a set of AT_ flags that can be used to control the scope of a given pathname lookup operation.

    There have been previous attempts at restricting pathname lookup, but none of them have been merged thus far. David Drysdale posted an O_BENEATH option to openat() in 2014 that would require the eventual target to be underneath the starting directory (as provided to openat()) in the filesystem hierarchy. More recently, Al Viro suggested AT_NO_JUMPS as a way of preventing lookups from venturing outside of the current directory hierarchy or the starting directory's mount point. Both ideas have attracted interest, but neither has yet been pushed long or hard enough to make it into the mainline.

  • Some numbers from the 4.19 development cycle

    The release of 4.19-rc6 on September 30 is an indication that the 4.19 development cycle is heading toward its conclusion. Naturally, that means it's time to have a look at where the contributions for this cycle came from. The upheavals currently playing out in the kernel community do not show at this level, but there are some new faces to be seen in the top contributors this time around.

    As of this writing, 13,657 non-merge changesets have found their way into the mainline for 4.19.

  • The modernization of PCIe hotplug in Linux

    PCI Express hotplug has been supported in Linux for fourteen years. The code, which is aging, is currently undergoing a transformation to fit the needs of contemporary applications such as hot-swappable flash drives in data centers and power-manageable Thunderbolt controllers in laptops. Time for a roundup.

    The initial PCI specification from 1992 had no provisions for the addition or removal of cards at runtime. In the late 1990s and early 2000s, various proprietary hotplug controllers, as well as the vendor-neutral standard hotplug controller, were conceived and became supported by Linux through drivers living in drivers/pci/hotplug. PCI Express (PCIe), instead, supported hotplug from the get-go in 2002, but its embodiments have changed over time. Originally intended to hot-swap PCIe cards in servers or ExpressCards in laptops, today it is commonly used in data centers (where NVMe flash drives need to be swapped at runtime) and by Thunderbolt (which tunnels PCIe through a hotpluggable chain of converged I/O switches, together with other protocols such as DisplayPort).

PCI Peer-To-Peer Memory Support Queued Ahead Of Linux 4.20~5.0

  • PCI Peer-To-Peer Memory Support Queued Ahead Of Linux 4.20~5.0

    With the upcoming Linux 4.20 kernel cycle (that given past comments by Linus Torvalds might be renamed to Linux 5.0), a new PCI feature queued ahead of the upcoming merge window is peer-to-peer memory support.

    This peer-to-peer (P2P) PCI memory support has been in the works for a while now. What this functionality is fundamentally about is supporting PCI (Express) devices that have memory mapped within their BAR space (Base Address Register) for peer-to-peer transactions. The new kernel code provides the necessary interface so other kernel subsystems can find/allocate portions of this memory as for peer-to-peer memory via P2P DMA. Peer-to-peer memory can help reduce pressure on the system RAM as memory is exchanged directly between PCIe devices.

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