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authorLibravatar Linus Torvalds <torvalds@linux-foundation.org>2023-02-21 18:24:12 -0800
committerLibravatar Linus Torvalds <torvalds@linux-foundation.org>2023-02-21 18:24:12 -0800
commit5b7c4cabbb65f5c469464da6c5f614cbd7f730f2 (patch)
treecc5c2d0a898769fd59549594fedb3ee6f84e59a0 /Documentation/admin-guide/kernel-per-CPU-kthreads.rst
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Merge tag 'net-next-6.3' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-nextgrafted
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(). ...
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+==========================================
+Reducing OS jitter due to per-cpu kthreads
+==========================================
+
+This document lists per-CPU kthreads in the Linux kernel and presents
+options to control their OS jitter. Note that non-per-CPU kthreads are
+not listed here. To reduce OS jitter from non-per-CPU kthreads, bind
+them to a "housekeeping" CPU dedicated to such work.
+
+References
+==========
+
+- Documentation/core-api/irq/irq-affinity.rst: Binding interrupts to sets of CPUs.
+
+- Documentation/admin-guide/cgroup-v1: Using cgroups to bind tasks to sets of CPUs.
+
+- man taskset: Using the taskset command to bind tasks to sets
+ of CPUs.
+
+- man sched_setaffinity: Using the sched_setaffinity() system
+ call to bind tasks to sets of CPUs.
+
+- /sys/devices/system/cpu/cpuN/online: Control CPU N's hotplug state,
+ writing "0" to offline and "1" to online.
+
+- In order to locate kernel-generated OS jitter on CPU N:
+
+ cd /sys/kernel/debug/tracing
+ echo 1 > max_graph_depth # Increase the "1" for more detail
+ echo function_graph > current_tracer
+ # run workload
+ cat per_cpu/cpuN/trace
+
+kthreads
+========
+
+Name:
+ ehca_comp/%u
+
+Purpose:
+ Periodically process Infiniband-related work.
+
+To reduce its OS jitter, do any of the following:
+
+1. Don't use eHCA Infiniband hardware, instead choosing hardware
+ that does not require per-CPU kthreads. This will prevent these
+ kthreads from being created in the first place. (This will
+ work for most people, as this hardware, though important, is
+ relatively old and is produced in relatively low unit volumes.)
+2. Do all eHCA-Infiniband-related work on other CPUs, including
+ interrupts.
+3. Rework the eHCA driver so that its per-CPU kthreads are
+ provisioned only on selected CPUs.
+
+
+Name:
+ irq/%d-%s
+
+Purpose:
+ Handle threaded interrupts.
+
+To reduce its OS jitter, do the following:
+
+1. Use irq affinity to force the irq threads to execute on
+ some other CPU.
+
+Name:
+ kcmtpd_ctr_%d
+
+Purpose:
+ Handle Bluetooth work.
+
+To reduce its OS jitter, do one of the following:
+
+1. Don't use Bluetooth, in which case these kthreads won't be
+ created in the first place.
+2. Use irq affinity to force Bluetooth-related interrupts to
+ occur on some other CPU and furthermore initiate all
+ Bluetooth activity on some other CPU.
+
+Name:
+ ksoftirqd/%u
+
+Purpose:
+ Execute softirq handlers when threaded or when under heavy load.
+
+To reduce its OS jitter, each softirq vector must be handled
+separately as follows:
+
+TIMER_SOFTIRQ
+-------------
+
+Do all of the following:
+
+1. To the extent possible, keep the CPU out of the kernel when it
+ is non-idle, for example, by avoiding system calls and by forcing
+ both kernel threads and interrupts to execute elsewhere.
+2. Build with CONFIG_HOTPLUG_CPU=y. After boot completes, force
+ the CPU offline, then bring it back online. This forces
+ recurring timers to migrate elsewhere. If you are concerned
+ with multiple CPUs, force them all offline before bringing the
+ first one back online. Once you have onlined the CPUs in question,
+ do not offline any other CPUs, because doing so could force the
+ timer back onto one of the CPUs in question.
+
+NET_TX_SOFTIRQ and NET_RX_SOFTIRQ
+---------------------------------
+
+Do all of the following:
+
+1. Force networking interrupts onto other CPUs.
+2. Initiate any network I/O on other CPUs.
+3. Once your application has started, prevent CPU-hotplug operations
+ from being initiated from tasks that might run on the CPU to
+ be de-jittered. (It is OK to force this CPU offline and then
+ bring it back online before you start your application.)
+
+BLOCK_SOFTIRQ
+-------------
+
+Do all of the following:
+
+1. Force block-device interrupts onto some other CPU.
+2. Initiate any block I/O on other CPUs.
+3. Once your application has started, prevent CPU-hotplug operations
+ from being initiated from tasks that might run on the CPU to
+ be de-jittered. (It is OK to force this CPU offline and then
+ bring it back online before you start your application.)
+
+IRQ_POLL_SOFTIRQ
+----------------
+
+Do all of the following:
+
+1. Force block-device interrupts onto some other CPU.
+2. Initiate any block I/O and block-I/O polling on other CPUs.
+3. Once your application has started, prevent CPU-hotplug operations
+ from being initiated from tasks that might run on the CPU to
+ be de-jittered. (It is OK to force this CPU offline and then
+ bring it back online before you start your application.)
+
+TASKLET_SOFTIRQ
+---------------
+
+Do one or more of the following:
+
+1. Avoid use of drivers that use tasklets. (Such drivers will contain
+ calls to things like tasklet_schedule().)
+2. Convert all drivers that you must use from tasklets to workqueues.
+3. Force interrupts for drivers using tasklets onto other CPUs,
+ and also do I/O involving these drivers on other CPUs.
+
+SCHED_SOFTIRQ
+-------------
+
+Do all of the following:
+
+1. Avoid sending scheduler IPIs to the CPU to be de-jittered,
+ for example, ensure that at most one runnable kthread is present
+ on that CPU. If a thread that expects to run on the de-jittered
+ CPU awakens, the scheduler will send an IPI that can result in
+ a subsequent SCHED_SOFTIRQ.
+2. CONFIG_NO_HZ_FULL=y and ensure that the CPU to be de-jittered
+ is marked as an adaptive-ticks CPU using the "nohz_full="
+ boot parameter. This reduces the number of scheduler-clock
+ interrupts that the de-jittered CPU receives, minimizing its
+ chances of being selected to do the load balancing work that
+ runs in SCHED_SOFTIRQ context.
+3. To the extent possible, keep the CPU out of the kernel when it
+ is non-idle, for example, by avoiding system calls and by
+ forcing both kernel threads and interrupts to execute elsewhere.
+ This further reduces the number of scheduler-clock interrupts
+ received by the de-jittered CPU.
+
+HRTIMER_SOFTIRQ
+---------------
+
+Do all of the following:
+
+1. To the extent possible, keep the CPU out of the kernel when it
+ is non-idle. For example, avoid system calls and force both
+ kernel threads and interrupts to execute elsewhere.
+2. Build with CONFIG_HOTPLUG_CPU=y. Once boot completes, force the
+ CPU offline, then bring it back online. This forces recurring
+ timers to migrate elsewhere. If you are concerned with multiple
+ CPUs, force them all offline before bringing the first one
+ back online. Once you have onlined the CPUs in question, do not
+ offline any other CPUs, because doing so could force the timer
+ back onto one of the CPUs in question.
+
+RCU_SOFTIRQ
+-----------
+
+Do at least one of the following:
+
+1. Offload callbacks and keep the CPU in either dyntick-idle or
+ adaptive-ticks state by doing all of the following:
+
+ a. CONFIG_NO_HZ_FULL=y and ensure that the CPU to be
+ de-jittered is marked as an adaptive-ticks CPU using the
+ "nohz_full=" boot parameter. Bind the rcuo kthreads to
+ housekeeping CPUs, which can tolerate OS jitter.
+ b. To the extent possible, keep the CPU out of the kernel
+ when it is non-idle, for example, by avoiding system
+ calls and by forcing both kernel threads and interrupts
+ to execute elsewhere.
+
+2. Enable RCU to do its processing remotely via dyntick-idle by
+ doing all of the following:
+
+ a. Build with CONFIG_NO_HZ=y.
+ b. Ensure that the CPU goes idle frequently, allowing other
+ CPUs to detect that it has passed through an RCU quiescent
+ state. If the kernel is built with CONFIG_NO_HZ_FULL=y,
+ userspace execution also allows other CPUs to detect that
+ the CPU in question has passed through a quiescent state.
+ c. To the extent possible, keep the CPU out of the kernel
+ when it is non-idle, for example, by avoiding system
+ calls and by forcing both kernel threads and interrupts
+ to execute elsewhere.
+
+Name:
+ kworker/%u:%d%s (cpu, id, priority)
+
+Purpose:
+ Execute workqueue requests
+
+To reduce its OS jitter, do any of the following:
+
+1. Run your workload at a real-time priority, which will allow
+ preempting the kworker daemons.
+2. A given workqueue can be made visible in the sysfs filesystem
+ by passing the WQ_SYSFS to that workqueue's alloc_workqueue().
+ Such a workqueue can be confined to a given subset of the
+ CPUs using the ``/sys/devices/virtual/workqueue/*/cpumask`` sysfs
+ files. The set of WQ_SYSFS workqueues can be displayed using
+ "ls /sys/devices/virtual/workqueue". That said, the workqueues
+ maintainer would like to caution people against indiscriminately
+ sprinkling WQ_SYSFS across all the workqueues. The reason for
+ caution is that it is easy to add WQ_SYSFS, but because sysfs is
+ part of the formal user/kernel API, it can be nearly impossible
+ to remove it, even if its addition was a mistake.
+3. Do any of the following needed to avoid jitter that your
+ application cannot tolerate:
+
+ a. Build your kernel with CONFIG_SLUB=y rather than
+ CONFIG_SLAB=y, thus avoiding the slab allocator's periodic
+ use of each CPU's workqueues to run its cache_reap()
+ function.
+ b. Avoid using oprofile, thus avoiding OS jitter from
+ wq_sync_buffer().
+ c. Limit your CPU frequency so that a CPU-frequency
+ governor is not required, possibly enlisting the aid of
+ special heatsinks or other cooling technologies. If done
+ correctly, and if you CPU architecture permits, you should
+ be able to build your kernel with CONFIG_CPU_FREQ=n to
+ avoid the CPU-frequency governor periodically running
+ on each CPU, including cs_dbs_timer() and od_dbs_timer().
+
+ WARNING: Please check your CPU specifications to
+ make sure that this is safe on your particular system.
+ d. As of v3.18, Christoph Lameter's on-demand vmstat workers
+ commit prevents OS jitter due to vmstat_update() on
+ CONFIG_SMP=y systems. Before v3.18, is not possible
+ to entirely get rid of the OS jitter, but you can
+ decrease its frequency by writing a large value to
+ /proc/sys/vm/stat_interval. The default value is HZ,
+ for an interval of one second. Of course, larger values
+ will make your virtual-memory statistics update more
+ slowly. Of course, you can also run your workload at
+ a real-time priority, thus preempting vmstat_update(),
+ but if your workload is CPU-bound, this is a bad idea.
+ However, there is an RFC patch from Christoph Lameter
+ (based on an earlier one from Gilad Ben-Yossef) that
+ reduces or even eliminates vmstat overhead for some
+ workloads at https://lore.kernel.org/r/00000140e9dfd6bd-40db3d4f-c1be-434f-8132-7820f81bb586-000000@email.amazonses.com.
+ e. If running on high-end powerpc servers, build with
+ CONFIG_PPC_RTAS_DAEMON=n. This prevents the RTAS
+ daemon from running on each CPU every second or so.
+ (This will require editing Kconfig files and will defeat
+ this platform's RAS functionality.) This avoids jitter
+ due to the rtas_event_scan() function.
+ WARNING: Please check your CPU specifications to
+ make sure that this is safe on your particular system.
+ f. If running on Cell Processor, build your kernel with
+ CBE_CPUFREQ_SPU_GOVERNOR=n to avoid OS jitter from
+ spu_gov_work().
+ WARNING: Please check your CPU specifications to
+ make sure that this is safe on your particular system.
+ g. If running on PowerMAC, build your kernel with
+ CONFIG_PMAC_RACKMETER=n to disable the CPU-meter,
+ avoiding OS jitter from rackmeter_do_timer().
+
+Name:
+ rcuc/%u
+
+Purpose:
+ Execute RCU callbacks in CONFIG_RCU_BOOST=y kernels.
+
+To reduce its OS jitter, do at least one of the following:
+
+1. Build the kernel with CONFIG_PREEMPT=n. This prevents these
+ kthreads from being created in the first place, and also obviates
+ the need for RCU priority boosting. This approach is feasible
+ for workloads that do not require high degrees of responsiveness.
+2. Build the kernel with CONFIG_RCU_BOOST=n. This prevents these
+ kthreads from being created in the first place. This approach
+ is feasible only if your workload never requires RCU priority
+ boosting, for example, if you ensure frequent idle time on all
+ CPUs that might execute within the kernel.
+3. Build with CONFIG_RCU_NOCB_CPU=y and boot with the rcu_nocbs=
+ boot parameter offloading RCU callbacks from all CPUs susceptible
+ to OS jitter. This approach prevents the rcuc/%u kthreads from
+ having any work to do, so that they are never awakened.
+4. Ensure that the CPU never enters the kernel, and, in particular,
+ avoid initiating any CPU hotplug operations on this CPU. This is
+ another way of preventing any callbacks from being queued on the
+ CPU, again preventing the rcuc/%u kthreads from having any work
+ to do.
+
+Name:
+ rcuop/%d and rcuos/%d
+
+Purpose:
+ Offload RCU callbacks from the corresponding CPU.
+
+To reduce its OS jitter, do at least one of the following:
+
+1. Use affinity, cgroups, or other mechanism to force these kthreads
+ to execute on some other CPU.
+2. Build with CONFIG_RCU_NOCB_CPU=n, which will prevent these
+ kthreads from being created in the first place. However, please
+ note that this will not eliminate OS jitter, but will instead
+ shift it to RCU_SOFTIRQ.