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Kernel: Latest Linux News From Phoronix, LWN, and Linus Torvalds Interview

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  • Multigen LRU Patches Updated For Addressing Linux's Expensive Page Reclaimation - Phoronix

    One of the interesting performance-related kernel patch series to come about so far this year has been Google's multi-generational LRU framework that is promising to offer much better performance in addressing the kernel's expensive page reclaimation handling.

    The Multigen LRU framework addresses issues with the current kernel design, "the current page reclaim is too expensive in terms of CPU usage and often making poor choices about what to evict. We would like to offer an alternative framework that is performant, versatile and straightforward." This patch series has been touted as leading to ~18% less low-memory kills on Android, code starts reduced by 16%, and Google's testing on Chrome OS led to ~96% fewer low-memory tab discards and 59% less out-of-memory kills thanks to this improved least recently used framework.

  • ASpeed AST2500 SSIF BMC Driver Being Worked On For The Linux Kernel - Phoronix

    ARM server vendor Ampere Computing has been working on an SSIF BMC-side driver for the Aspeed ASTC2500 baseboard management controller that they are looking to upstream in the Linux kernel.

    SSIF is short for the SMBus System Interface and allows accessing the baseboard management controller by way of the SMBus host controller.

  • AMD "Beige Goby" GPU Support Targeting Linux 5.14 Inclusion - Phoronix

    AMD submitted another set of feature changes to DRM-Next for queuing ahead of this summer's Linux 5.14 cycle.

    This latest pull request just comes one week after their initial 5.14 pull last week that included more work on their Adebaran CDNA accelerator, initial HHVM SVM support, DP LTTPR handling improvements, PCI Express ASPM enablement, TMZ for Renoir, support for handling multiple eDP panels, and other enhancements.

  • New Patches Posted For Linux Runtime Verification

    A new patch series has been posted implementing Runtime Verification (RV) for the Linux kernel.

  • The second half of the 5.13 merge window [LWN.net]

    By the time the last pull request was acted on and 5.13-rc1 was released, a total of 14,231 non-merge commits had found their way into the mainline. That makes the 5.13 merge window larger than the entire 5.12 development cycle (13,015 commits) and just short of all of 5.11 (14,340). In other words, 5.13 looks like one of the busier development cycles we have seen for a little while. About 6,400 of these commits came in after the first-half summary was written, and they include a number of significant new features.

  • Noncoherent DMA mappings [LWN.net]

    While it is sometimes possible to perform I/O by moving data through the CPU, the only way to get the required level of performance is usually for devices to move data directly to and from memory. Direct memory access (DMA) I/O has been well supported in the Linux kernel since the early days, but there are always ways in which that support can be improved, especially when hardware adds some challenges of its own. The somewhat confusingly named "non-contiguous" DMA API that was added for 5.13 shows the kinds of things that have to be done to get the best performance on current systems.

    DMA, of course, presents a number of interesting race conditions that can arise in the absence of an agreement between the CPU and the device over who controls a range of memory at any given time. But there is another problem that comes up as well. CPUs aggressively cache memory contents to avoid the considerable expense of actually going to memory for every reference. But if a CPU has cached data that is subsequently overwritten by DMA, the CPU could end up reading incorrect data from the cache, resulting in data corruption. Similarly, if the cache contains data written by the CPU that has not yet made it to memory, that data really needs to be flushed out before the device accesses that memory or bad things are likely to happen.

    The x86 architecture makes life relatively easy (in this regard, at least) for kernel developers by providing cache snooping; CPU caches will be invalidated if a device is seen to be writing to a range of memory, for example. This "cache-coherent" behavior means that developers need not worry about cache contents corrupting their data. Other architectures are not so forgiving. The Arm architecture, among others, will happily retain cache contents that no longer match the memory they are allegedly caching. On such systems, developers must take care to manage the cache properly as control of a DMA buffer is passed between the device and the CPU.

  • A pair of memory-allocation improvements in 5.13

    Among the many changes merged for 5.13 can be found performance improvements throughout the kernel. This work does not always stand out the way that new features do, but it is vitally important for the future of the kernel overall. In the memory-management area, a couple of long-running patch sets have finally made it into the mainline; these provide a bulk page-allocation interface and huge-page mappings in the vmalloc() area. Both of these changes should make things faster, at least for some workloads.

    Batch page allocation

    The kernel's memory-allocation functions have long been optimized for performance and scalability, but there are situations where that work still has not achieved the desired results. One of those is high-speed networking. Back in 2016, networking developer Jesper Dangaard Brouer described the challenges that come with the fastest network links; when the system is processing tens of millions of packets per second, the time available to deal with any given packet is highly constrained. The kernel may only have a few hundred CPU cycles available to process each packet, and obtaining a page from the memory allocator may, by itself, require more than that. Using the entire CPU-time budget to allocate memory is not the way to get the best network performance.

    At the time, Brouer asked for an API that would allow numerous pages to be allocated with a single call, hopefully with a much lower per-page cost. The networking code could then grab a pile of memory, and quickly hand out pages as needed. Nobody objected to the request at the time; it is well understood that batching operations can increase throughput in situations like this. But it took some time for that interface to come around.

  • Linus Torvalds on the Importance of Open Source

    Ahead of the 30th anniversary of Linux, coming up in August of 2021, Jeremy Andrews has conducted an two-part, in-depth interview with Linus Torvalds, covering topics such as open source licensing, corporate contributions, Git, project maintenance, and more.

    When asked about choosing the GPLv2 license for the Linux kernel, Torvalds says, “I’m 100% convinced that the license has been a big part of the success of Linux (and Git, for that matter). I think everybody involved ends up being much happier when they know that everybody has equal rights, and nobody is special with regards to licensing.”

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