4.12 Merge window part 1

Some of the more interesting user-visible changes merged thus far include:

• As expected, the BFQ and Kyber block I/O schedulers have been merged. The kernel now has two multiqueue I/O schedulers suitable for widely varying use cases, and the long wait for BFQ in the mainline is at an end.

• The blk-throttle control-group controller has a new "low" limit that serves as a sort of soft cap. No group is allowed to exceed its low limit until all active groups have reached their respective low limits. It is also now possible to adjust the sample period used by the controller, trading off fine control against CPU overhead.

• The LightNVM subsystem has gained a "pblk" target which will expose an open-channel SSD as an ordinary-looking block device.

• The prctl() system call has two new operations: ARCH_SET_CPUID to allow trapping of the CPUID instruction, and ARCH_GET_CPUID to get the current state of that trapping. This feature, which is only implemented on the x86 architecture, is expected to be useful for tracing applications that want to trap and emulate this instruction.

• As usual, the perf events subsystem has seen a number of changes; see this merge commit for the list.

• The BPF virtual machine subsystem has seen a few improvements. Maps are now able to contain other maps, allowing them to be cascaded to multiple levels. There is a new in-kernel testing framework for BPF programs, controlled by the new BPF_PROG_TEST_RUN command to the bpf() system call. And there is now a just-in-time BPF compiler for the SPARC64 architecture.

• The epoll_wait() system call can now perform busy-polling of network sockets, reducing packet-reception latencies.

• The "hybrid consistency model" for live kernel patching has been merged. This model, discussed in this article, enables the application of patches that change function or data semantics. See this commit for an overview of how it works.

• The MD RAID5 implementation has gained "partial parity log" support. This feature can reduce the possibility of corruption when running with a degraded array. See Documentation/md/raid5-ppl.txt for more information.

• The device mapper supports a new dm-integrity target; it emulates a device with extra per-sector integrity tags. See Documentation/device-mapper/dm-integrity.txt for details.

• New hardware support includes:
  • Cryptographic: Cavium ThunderX "ZIP" compression engines, Freescale CAAM Queue-Interface crypto engines, STMicroelectronis STM32 crypto accelerators, and Mediatek random number generators.

  • Input: Accutouch 2216 touch controllers.

  • Miscellaneous: ASPEED AST2400/AST2500 PWM and fan controllers, Motorola CPCAP PMIC battery chargers, LEGO MINDSTORMS EV3 batteries, Broadcom FlexRM ring managers, Mediatek MT6323 PMIC LED controllers, Motorola CPCAP PMIC LED controllers, DaVinci DM816 AHCI SATA controllers, and NVIDIA Tegra186 CPU-frequency controllers.

  • Multi-media card: Broadcom BCM2835 SDHOST MMC controllers, Cavium ThunderX and Octeon SD/MMC card interfaces, and Marvell Xenon eMMC/SD/SDIO SDHCI interfaces.

  • Networking: APM X-Gene SoC Ethernet interfaces, Synopsys DWC Enterprise Ethernet adapters, Holt HI311x SPI CAN controllers, Cascoda CA8210 SPI 802.15.4 wireless controllers, SMSC/MicroChip LAN9303 three-port Ethernet switches, and Microchip CAN bus analyzer interfaces.

  • Pin control: Axis ARTPEC-6 pin controllers, Marvell 37xx SoC pin controllers, and STMicroelectronics STM32F469 pin controllers.

Changes visible to kernel developers include:

• The "hd" disk driver, written by Linus and present since the 0.01 release, has been removed at last.

• The new "AnalyzeBoot" tool can create a timeline of the kernel's bootstrap process in HTML format.

• The code for accessing user-space data from the kernel has been significantly reworked, resulting in the removal of a lot of architecture-specific code.

• The AVR32 architecture has been removed from the kernel. The chips have been past their end of life for some time, and the kernel code has been poorly maintained at best.

• The "generic XDP" functionality in the networking stack implements express data path functionality on devices that lack their own optimized implementation. It is meant to make XDP functionality more widely available, especially for developers who are new to it.

The 4.12 merge window will likely remain open through May 14, and the 4.12 release will probably happen in early July. As always, LWN will continue to follow the patch stream as this merge window runs its course.