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author | 2023-02-21 18:24:12 -0800 | |
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committer | 2023-02-21 18:24:12 -0800 | |
commit | 5b7c4cabbb65f5c469464da6c5f614cbd7f730f2 (patch) | |
tree | cc5c2d0a898769fd59549594fedb3ee6f84e59a0 /Documentation/scheduler/completion.rst | |
download | linux-5b7c4cabbb65f5c469464da6c5f614cbd7f730f2.tar.gz linux-5b7c4cabbb65f5c469464da6c5f614cbd7f730f2.zip |
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().
...
Diffstat (limited to 'Documentation/scheduler/completion.rst')
-rw-r--r-- | Documentation/scheduler/completion.rst | 293 |
1 files changed, 293 insertions, 0 deletions
diff --git a/Documentation/scheduler/completion.rst b/Documentation/scheduler/completion.rst new file mode 100644 index 000000000..9f039b4f4 --- /dev/null +++ b/Documentation/scheduler/completion.rst @@ -0,0 +1,293 @@ +================================================ +Completions - "wait for completion" barrier APIs +================================================ + +Introduction: +------------- + +If you have one or more threads that must wait for some kernel activity +to have reached a point or a specific state, completions can provide a +race-free solution to this problem. Semantically they are somewhat like a +pthread_barrier() and have similar use-cases. + +Completions are a code synchronization mechanism which is preferable to any +misuse of locks/semaphores and busy-loops. Any time you think of using +yield() or some quirky msleep(1) loop to allow something else to proceed, +you probably want to look into using one of the wait_for_completion*() +calls and complete() instead. + +The advantage of using completions is that they have a well defined, focused +purpose which makes it very easy to see the intent of the code, but they +also result in more efficient code as all threads can continue execution +until the result is actually needed, and both the waiting and the signalling +is highly efficient using low level scheduler sleep/wakeup facilities. + +Completions are built on top of the waitqueue and wakeup infrastructure of +the Linux scheduler. The event the threads on the waitqueue are waiting for +is reduced to a simple flag in 'struct completion', appropriately called "done". + +As completions are scheduling related, the code can be found in +kernel/sched/completion.c. + + +Usage: +------ + +There are three main parts to using completions: + + - the initialization of the 'struct completion' synchronization object + - the waiting part through a call to one of the variants of wait_for_completion(), + - the signaling side through a call to complete() or complete_all(). + +There are also some helper functions for checking the state of completions. +Note that while initialization must happen first, the waiting and signaling +part can happen in any order. I.e. it's entirely normal for a thread +to have marked a completion as 'done' before another thread checks whether +it has to wait for it. + +To use completions you need to #include <linux/completion.h> and +create a static or dynamic variable of type 'struct completion', +which has only two fields:: + + struct completion { + unsigned int done; + wait_queue_head_t wait; + }; + +This provides the ->wait waitqueue to place tasks on for waiting (if any), and +the ->done completion flag for indicating whether it's completed or not. + +Completions should be named to refer to the event that is being synchronized on. +A good example is:: + + wait_for_completion(&early_console_added); + + complete(&early_console_added); + +Good, intuitive naming (as always) helps code readability. Naming a completion +'complete' is not helpful unless the purpose is super obvious... + + +Initializing completions: +------------------------- + +Dynamically allocated completion objects should preferably be embedded in data +structures that are assured to be alive for the life-time of the function/driver, +to prevent races with asynchronous complete() calls from occurring. + +Particular care should be taken when using the _timeout() or _killable()/_interruptible() +variants of wait_for_completion(), as it must be assured that memory de-allocation +does not happen until all related activities (complete() or reinit_completion()) +have taken place, even if these wait functions return prematurely due to a timeout +or a signal triggering. + +Initializing of dynamically allocated completion objects is done via a call to +init_completion():: + + init_completion(&dynamic_object->done); + +In this call we initialize the waitqueue and set ->done to 0, i.e. "not completed" +or "not done". + +The re-initialization function, reinit_completion(), simply resets the +->done field to 0 ("not done"), without touching the waitqueue. +Callers of this function must make sure that there are no racy +wait_for_completion() calls going on in parallel. + +Calling init_completion() on the same completion object twice is +most likely a bug as it re-initializes the queue to an empty queue and +enqueued tasks could get "lost" - use reinit_completion() in that case, +but be aware of other races. + +For static declaration and initialization, macros are available. + +For static (or global) declarations in file scope you can use +DECLARE_COMPLETION():: + + static DECLARE_COMPLETION(setup_done); + DECLARE_COMPLETION(setup_done); + +Note that in this case the completion is boot time (or module load time) +initialized to 'not done' and doesn't require an init_completion() call. + +When a completion is declared as a local variable within a function, +then the initialization should always use DECLARE_COMPLETION_ONSTACK() +explicitly, not just to make lockdep happy, but also to make it clear +that limited scope had been considered and is intentional:: + + DECLARE_COMPLETION_ONSTACK(setup_done) + +Note that when using completion objects as local variables you must be +acutely aware of the short life time of the function stack: the function +must not return to a calling context until all activities (such as waiting +threads) have ceased and the completion object is completely unused. + +To emphasise this again: in particular when using some of the waiting API variants +with more complex outcomes, such as the timeout or signalling (_timeout(), +_killable() and _interruptible()) variants, the wait might complete +prematurely while the object might still be in use by another thread - and a return +from the wait_on_completion*() caller function will deallocate the function +stack and cause subtle data corruption if a complete() is done in some +other thread. Simple testing might not trigger these kinds of races. + +If unsure, use dynamically allocated completion objects, preferably embedded +in some other long lived object that has a boringly long life time which +exceeds the life time of any helper threads using the completion object, +or has a lock or other synchronization mechanism to make sure complete() +is not called on a freed object. + +A naive DECLARE_COMPLETION() on the stack triggers a lockdep warning. + +Waiting for completions: +------------------------ + +For a thread to wait for some concurrent activity to finish, it +calls wait_for_completion() on the initialized completion structure:: + + void wait_for_completion(struct completion *done) + +A typical usage scenario is:: + + CPU#1 CPU#2 + + struct completion setup_done; + + init_completion(&setup_done); + initialize_work(...,&setup_done,...); + + /* run non-dependent code */ /* do setup */ + + wait_for_completion(&setup_done); complete(setup_done); + +This is not implying any particular order between wait_for_completion() and +the call to complete() - if the call to complete() happened before the call +to wait_for_completion() then the waiting side simply will continue +immediately as all dependencies are satisfied; if not, it will block until +completion is signaled by complete(). + +Note that wait_for_completion() is calling spin_lock_irq()/spin_unlock_irq(), +so it can only be called safely when you know that interrupts are enabled. +Calling it from IRQs-off atomic contexts will result in hard-to-detect +spurious enabling of interrupts. + +The default behavior is to wait without a timeout and to mark the task as +uninterruptible. wait_for_completion() and its variants are only safe +in process context (as they can sleep) but not in atomic context, +interrupt context, with disabled IRQs, or preemption is disabled - see also +try_wait_for_completion() below for handling completion in atomic/interrupt +context. + +As all variants of wait_for_completion() can (obviously) block for a long +time depending on the nature of the activity they are waiting for, so in +most cases you probably don't want to call this with held mutexes. + + +wait_for_completion*() variants available: +------------------------------------------ + +The below variants all return status and this status should be checked in +most(/all) cases - in cases where the status is deliberately not checked you +probably want to make a note explaining this (e.g. see +arch/arm/kernel/smp.c:__cpu_up()). + +A common problem that occurs is to have unclean assignment of return types, +so take care to assign return-values to variables of the proper type. + +Checking for the specific meaning of return values also has been found +to be quite inaccurate, e.g. constructs like:: + + if (!wait_for_completion_interruptible_timeout(...)) + +... would execute the same code path for successful completion and for the +interrupted case - which is probably not what you want:: + + int wait_for_completion_interruptible(struct completion *done) + +This function marks the task TASK_INTERRUPTIBLE while it is waiting. +If a signal was received while waiting it will return -ERESTARTSYS; 0 otherwise:: + + unsigned long wait_for_completion_timeout(struct completion *done, unsigned long timeout) + +The task is marked as TASK_UNINTERRUPTIBLE and will wait at most 'timeout' +jiffies. If a timeout occurs it returns 0, else the remaining time in +jiffies (but at least 1). + +Timeouts are preferably calculated with msecs_to_jiffies() or usecs_to_jiffies(), +to make the code largely HZ-invariant. + +If the returned timeout value is deliberately ignored a comment should probably explain +why (e.g. see drivers/mfd/wm8350-core.c wm8350_read_auxadc()):: + + long wait_for_completion_interruptible_timeout(struct completion *done, unsigned long timeout) + +This function passes a timeout in jiffies and marks the task as +TASK_INTERRUPTIBLE. If a signal was received it will return -ERESTARTSYS; +otherwise it returns 0 if the completion timed out, or the remaining time in +jiffies if completion occurred. + +Further variants include _killable which uses TASK_KILLABLE as the +designated tasks state and will return -ERESTARTSYS if it is interrupted, +or 0 if completion was achieved. There is a _timeout variant as well:: + + long wait_for_completion_killable(struct completion *done) + long wait_for_completion_killable_timeout(struct completion *done, unsigned long timeout) + +The _io variants wait_for_completion_io() behave the same as the non-_io +variants, except for accounting waiting time as 'waiting on IO', which has +an impact on how the task is accounted in scheduling/IO stats:: + + void wait_for_completion_io(struct completion *done) + unsigned long wait_for_completion_io_timeout(struct completion *done, unsigned long timeout) + + +Signaling completions: +---------------------- + +A thread that wants to signal that the conditions for continuation have been +achieved calls complete() to signal exactly one of the waiters that it can +continue:: + + void complete(struct completion *done) + +... or calls complete_all() to signal all current and future waiters:: + + void complete_all(struct completion *done) + +The signaling will work as expected even if completions are signaled before +a thread starts waiting. This is achieved by the waiter "consuming" +(decrementing) the done field of 'struct completion'. Waiting threads +wakeup order is the same in which they were enqueued (FIFO order). + +If complete() is called multiple times then this will allow for that number +of waiters to continue - each call to complete() will simply increment the +done field. Calling complete_all() multiple times is a bug though. Both +complete() and complete_all() can be called in IRQ/atomic context safely. + +There can only be one thread calling complete() or complete_all() on a +particular 'struct completion' at any time - serialized through the wait +queue spinlock. Any such concurrent calls to complete() or complete_all() +probably are a design bug. + +Signaling completion from IRQ context is fine as it will appropriately +lock with spin_lock_irqsave()/spin_unlock_irqrestore() and it will never +sleep. + + +try_wait_for_completion()/completion_done(): +-------------------------------------------- + +The try_wait_for_completion() function will not put the thread on the wait +queue but rather returns false if it would need to enqueue (block) the thread, +else it consumes one posted completion and returns true:: + + bool try_wait_for_completion(struct completion *done) + +Finally, to check the state of a completion without changing it in any way, +call completion_done(), which returns false if there are no posted +completions that were not yet consumed by waiters (implying that there are +waiters) and true otherwise:: + + bool completion_done(struct completion *done) + +Both try_wait_for_completion() and completion_done() are safe to be called in +IRQ or atomic context. |