From 5b7c4cabbb65f5c469464da6c5f614cbd7f730f2 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Tue, 21 Feb 2023 18:24:12 -0800 Subject: Merge tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next Pull networking updates from Jakub Kicinski: "Core: - Add dedicated kmem_cache for typical/small skb->head, avoid having to access struct page at kfree time, and improve memory use. - Introduce sysctl to set default RPS configuration for new netdevs. - Define Netlink protocol specification format which can be used to describe messages used by each family and auto-generate parsers. Add tools for generating kernel data structures and uAPI headers. - Expose all net/core sysctls inside netns. - Remove 4s sleep in netpoll if carrier is instantly detected on boot. - Add configurable limit of MDB entries per port, and port-vlan. - Continue populating drop reasons throughout the stack. - Retire a handful of legacy Qdiscs and classifiers. Protocols: - Support IPv4 big TCP (TSO frames larger than 64kB). - Add IP_LOCAL_PORT_RANGE socket option, to control local port range on socket by socket basis. - Track and report in procfs number of MPTCP sockets used. - Support mixing IPv4 and IPv6 flows in the in-kernel MPTCP path manager. - IPv6: don't check net.ipv6.route.max_size and rely on garbage collection to free memory (similarly to IPv4). - Support Penultimate Segment Pop (PSP) flavor in SRv6 (RFC8986). - ICMP: add per-rate limit counters. - Add support for user scanning requests in ieee802154. - Remove static WEP support. - Support minimal Wi-Fi 7 Extremely High Throughput (EHT) rate reporting. - WiFi 7 EHT channel puncturing support (client & AP). BPF: - Add a rbtree data structure following the "next-gen data structure" precedent set by recently added linked list, that is, by using kfunc + kptr instead of adding a new BPF map type. - Expose XDP hints via kfuncs with initial support for RX hash and timestamp metadata. - Add BPF_F_NO_TUNNEL_KEY extension to bpf_skb_set_tunnel_key to better support decap on GRE tunnel devices not operating in collect metadata. - Improve x86 JIT's codegen for PROBE_MEM runtime error checks. - Remove the need for trace_printk_lock for bpf_trace_printk and bpf_trace_vprintk helpers. - Extend libbpf's bpf_tracing.h support for tracing arguments of kprobes/uprobes and syscall as a special case. - Significantly reduce the search time for module symbols by livepatch and BPF. - Enable cpumasks to be used as kptrs, which is useful for tracing programs tracking which tasks end up running on which CPUs in different time intervals. - Add support for BPF trampoline on s390x and riscv64. - Add capability to export the XDP features supported by the NIC. - Add __bpf_kfunc tag for marking kernel functions as kfuncs. - Add cgroup.memory=nobpf kernel parameter option to disable BPF memory accounting for container environments. Netfilter: - Remove the CLUSTERIP target. It has been marked as obsolete for years, and we still have WARN splats wrt races of the out-of-band /proc interface installed by this target. - Add 'destroy' commands to nf_tables. They are identical to the existing 'delete' commands, but do not return an error if the referenced object (set, chain, rule...) did not exist. Driver API: - Improve cpumask_local_spread() locality to help NICs set the right IRQ affinity on AMD platforms. - Separate C22 and C45 MDIO bus transactions more clearly. - Introduce new DCB table to control DSCP rewrite on egress. - Support configuration of Physical Layer Collision Avoidance (PLCA) Reconciliation Sublayer (RS) (802.3cg-2019). Modern version of shared medium Ethernet. - Support for MAC Merge layer (IEEE 802.3-2018 clause 99). Allowing preemption of low priority frames by high priority frames. - Add support for controlling MACSec offload using netlink SET. - Rework devlink instance refcounts to allow registration and de-registration under the instance lock. Split the code into multiple files, drop some of the unnecessarily granular locks and factor out common parts of netlink operation handling. - Add TX frame aggregation parameters (for USB drivers). - Add a new attr TCA_EXT_WARN_MSG to report TC (offload) warning messages with notifications for debug. - Allow offloading of UDP NEW connections via act_ct. - Add support for per action HW stats in TC. - Support hardware miss to TC action (continue processing in SW from a specific point in the action chain). - Warn if old Wireless Extension user space interface is used with modern cfg80211/mac80211 drivers. Do not support Wireless Extensions for Wi-Fi 7 devices at all. Everyone should switch to using nl80211 interface instead. - Improve the CAN bit timing configuration. Use extack to return error messages directly to user space, update the SJW handling, including the definition of a new default value that will benefit CAN-FD controllers, by increasing their oscillator tolerance. New hardware / drivers: - Ethernet: - nVidia BlueField-3 support (control traffic driver) - Ethernet support for imx93 SoCs - Motorcomm yt8531 gigabit Ethernet PHY - onsemi NCN26000 10BASE-T1S PHY (with support for PLCA) - Microchip LAN8841 PHY (incl. cable diagnostics and PTP) - Amlogic gxl MDIO mux - WiFi: - RealTek RTL8188EU (rtl8xxxu) - Qualcomm Wi-Fi 7 devices (ath12k) - CAN: - Renesas R-Car V4H Drivers: - Bluetooth: - Set Per Platform Antenna Gain (PPAG) for Intel controllers. - Ethernet NICs: - Intel (1G, igc): - support TSN / Qbv / packet scheduling features of i226 model - Intel (100G, ice): - use GNSS subsystem instead of TTY - multi-buffer XDP support - extend support for GPIO pins to E823 devices - nVidia/Mellanox: - update the shared buffer configuration on PFC commands - implement PTP adjphase function for HW offset control - TC support for Geneve and GRE with VF tunnel offload - more efficient crypto key management method - multi-port eswitch support - Netronome/Corigine: - add DCB IEEE support - support IPsec offloading for NFP3800 - Freescale/NXP (enetc): - support XDP_REDIRECT for XDP non-linear buffers - improve reconfig, avoid link flap and waiting for idle - support MAC Merge layer - Other NICs: - sfc/ef100: add basic devlink support for ef100 - ionic: rx_push mode operation (writing descriptors via MMIO) - bnxt: use the auxiliary bus abstraction for RDMA - r8169: disable ASPM and reset bus in case of tx timeout - cpsw: support QSGMII mode for J721e CPSW9G - cpts: support pulse-per-second output - ngbe: add an mdio bus driver - usbnet: optimize usbnet_bh() by avoiding unnecessary queuing - r8152: handle devices with FW with NCM support - amd-xgbe: support 10Mbps, 2.5GbE speeds and rx-adaptation - virtio-net: support multi buffer XDP - virtio/vsock: replace virtio_vsock_pkt with sk_buff - tsnep: XDP support - Ethernet high-speed switches: - nVidia/Mellanox (mlxsw): - add support for latency TLV (in FW control messages) - Microchip (sparx5): - separate explicit and implicit traffic forwarding rules, make the implicit rules always active - add support for egress DSCP rewrite - IS0 VCAP support (Ingress Classification) - IS2 VCAP filters (protos, L3 addrs, L4 ports, flags, ToS etc.) - ES2 VCAP support (Egress Access Control) - support for Per-Stream Filtering and Policing (802.1Q, 8.6.5.1) - Ethernet embedded switches: - Marvell (mv88e6xxx): - add MAB (port auth) offload support - enable PTP receive for mv88e6390 - NXP (ocelot): - support MAC Merge layer - support for the the vsc7512 internal copper phys - Microchip: - lan9303: convert to PHYLINK - lan966x: support TC flower filter statistics - lan937x: PTP support for KSZ9563/KSZ8563 and LAN937x - lan937x: support Credit Based Shaper configuration - ksz9477: support Energy Efficient Ethernet - other: - qca8k: convert to regmap read/write API, use bulk operations - rswitch: Improve TX timestamp accuracy - Intel WiFi (iwlwifi): - EHT (Wi-Fi 7) rate reporting - STEP equalizer support: transfer some STEP (connection to radio on platforms with integrated wifi) related parameters from the BIOS to the firmware. - Qualcomm 802.11ax WiFi (ath11k): - IPQ5018 support - Fine Timing Measurement (FTM) responder role support - channel 177 support - MediaTek WiFi (mt76): - per-PHY LED support - mt7996: EHT (Wi-Fi 7) support - Wireless Ethernet Dispatch (WED) reset support - switch to using page pool allocator - RealTek WiFi (rtw89): - support new version of Bluetooth co-existance - Mobile: - rmnet: support TX aggregation" * tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next: (1872 commits) page_pool: add a comment explaining the fragment counter usage net: ethtool: fix __ethtool_dev_mm_supported() implementation ethtool: pse-pd: Fix double word in comments xsk: add linux/vmalloc.h to xsk.c sefltests: netdevsim: wait for devlink instance after netns removal selftest: fib_tests: Always cleanup before exit net/mlx5e: Align IPsec ASO result memory to be as required by hardware net/mlx5e: TC, Set CT miss to the specific ct action instance net/mlx5e: Rename CHAIN_TO_REG to MAPPED_OBJ_TO_REG net/mlx5: Refactor tc miss handling to a single function net/mlx5: Kconfig: Make tc offload depend on tc skb extension net/sched: flower: Support hardware miss to tc action net/sched: flower: Move filter handle initialization earlier net/sched: cls_api: Support hardware miss to tc action net/sched: Rename user cookie and act cookie sfc: fix builds without CONFIG_RTC_LIB sfc: clean up some inconsistent indentings net/mlx4_en: Introduce flexible array to silence overflow warning net: lan966x: Fix possible deadlock inside PTP net/ulp: Remove redundant ->clone() test in inet_clone_ulp(). ... --- Documentation/locking/pi-futex.rst | 122 +++++++++++++++++++++++++++++++++++++ 1 file changed, 122 insertions(+) create mode 100644 Documentation/locking/pi-futex.rst (limited to 'Documentation/locking/pi-futex.rst') diff --git a/Documentation/locking/pi-futex.rst b/Documentation/locking/pi-futex.rst new file mode 100644 index 000000000..c33ba2bef --- /dev/null +++ b/Documentation/locking/pi-futex.rst @@ -0,0 +1,122 @@ +====================== +Lightweight PI-futexes +====================== + +We are calling them lightweight for 3 reasons: + + - in the user-space fastpath a PI-enabled futex involves no kernel work + (or any other PI complexity) at all. No registration, no extra kernel + calls - just pure fast atomic ops in userspace. + + - even in the slowpath, the system call and scheduling pattern is very + similar to normal futexes. + + - the in-kernel PI implementation is streamlined around the mutex + abstraction, with strict rules that keep the implementation + relatively simple: only a single owner may own a lock (i.e. no + read-write lock support), only the owner may unlock a lock, no + recursive locking, etc. + +Priority Inheritance - why? +--------------------------- + +The short reply: user-space PI helps achieving/improving determinism for +user-space applications. In the best-case, it can help achieve +determinism and well-bound latencies. Even in the worst-case, PI will +improve the statistical distribution of locking related application +delays. + +The longer reply +---------------- + +Firstly, sharing locks between multiple tasks is a common programming +technique that often cannot be replaced with lockless algorithms. As we +can see it in the kernel [which is a quite complex program in itself], +lockless structures are rather the exception than the norm - the current +ratio of lockless vs. locky code for shared data structures is somewhere +between 1:10 and 1:100. Lockless is hard, and the complexity of lockless +algorithms often endangers to ability to do robust reviews of said code. +I.e. critical RT apps often choose lock structures to protect critical +data structures, instead of lockless algorithms. Furthermore, there are +cases (like shared hardware, or other resource limits) where lockless +access is mathematically impossible. + +Media players (such as Jack) are an example of reasonable application +design with multiple tasks (with multiple priority levels) sharing +short-held locks: for example, a highprio audio playback thread is +combined with medium-prio construct-audio-data threads and low-prio +display-colory-stuff threads. Add video and decoding to the mix and +we've got even more priority levels. + +So once we accept that synchronization objects (locks) are an +unavoidable fact of life, and once we accept that multi-task userspace +apps have a very fair expectation of being able to use locks, we've got +to think about how to offer the option of a deterministic locking +implementation to user-space. + +Most of the technical counter-arguments against doing priority +inheritance only apply to kernel-space locks. But user-space locks are +different, there we cannot disable interrupts or make the task +non-preemptible in a critical section, so the 'use spinlocks' argument +does not apply (user-space spinlocks have the same priority inversion +problems as other user-space locking constructs). Fact is, pretty much +the only technique that currently enables good determinism for userspace +locks (such as futex-based pthread mutexes) is priority inheritance: + +Currently (without PI), if a high-prio and a low-prio task shares a lock +[this is a quite common scenario for most non-trivial RT applications], +even if all critical sections are coded carefully to be deterministic +(i.e. all critical sections are short in duration and only execute a +limited number of instructions), the kernel cannot guarantee any +deterministic execution of the high-prio task: any medium-priority task +could preempt the low-prio task while it holds the shared lock and +executes the critical section, and could delay it indefinitely. + +Implementation +-------------- + +As mentioned before, the userspace fastpath of PI-enabled pthread +mutexes involves no kernel work at all - they behave quite similarly to +normal futex-based locks: a 0 value means unlocked, and a value==TID +means locked. (This is the same method as used by list-based robust +futexes.) Userspace uses atomic ops to lock/unlock these mutexes without +entering the kernel. + +To handle the slowpath, we have added two new futex ops: + + - FUTEX_LOCK_PI + - FUTEX_UNLOCK_PI + +If the lock-acquire fastpath fails, [i.e. an atomic transition from 0 to +TID fails], then FUTEX_LOCK_PI is called. The kernel does all the +remaining work: if there is no futex-queue attached to the futex address +yet then the code looks up the task that owns the futex [it has put its +own TID into the futex value], and attaches a 'PI state' structure to +the futex-queue. The pi_state includes an rt-mutex, which is a PI-aware, +kernel-based synchronization object. The 'other' task is made the owner +of the rt-mutex, and the FUTEX_WAITERS bit is atomically set in the +futex value. Then this task tries to lock the rt-mutex, on which it +blocks. Once it returns, it has the mutex acquired, and it sets the +futex value to its own TID and returns. Userspace has no other work to +perform - it now owns the lock, and futex value contains +FUTEX_WAITERS|TID. + +If the unlock side fastpath succeeds, [i.e. userspace manages to do a +TID -> 0 atomic transition of the futex value], then no kernel work is +triggered. + +If the unlock fastpath fails (because the FUTEX_WAITERS bit is set), +then FUTEX_UNLOCK_PI is called, and the kernel unlocks the futex on the +behalf of userspace - and it also unlocks the attached +pi_state->rt_mutex and thus wakes up any potential waiters. + +Note that under this approach, contrary to previous PI-futex approaches, +there is no prior 'registration' of a PI-futex. [which is not quite +possible anyway, due to existing ABI properties of pthread mutexes.] + +Also, under this scheme, 'robustness' and 'PI' are two orthogonal +properties of futexes, and all four combinations are possible: futex, +robust-futex, PI-futex, robust+PI-futex. + +More details about priority inheritance can be found in +Documentation/locking/rt-mutex.rst. -- cgit v1.2.3