NAPI

NAPI is the event handling mechanism used by the Linux networking stack. The name NAPI no longer stands for anything in particular [1].

In basic operation the device notifies the host about new events via an interrupt. The host then schedules a NAPI instance to process the events. The device may also be polled for events via NAPI without receiving interrupts first (busy polling).

NAPI processing usually happens in the software interrupt context, but there is an option to use separate kernel threads for NAPI processing.

All in all NAPI abstracts away from the drivers the context and configuration of event (packet Rx and Tx) processing.

Driver API

The two most important elements of NAPI are the struct napi_struct and the associated poll method. struct napi_struct holds the state of the NAPI instance while the method is the driver-specific event handler. The method will typically free Tx packets that have been transmitted and process newly received packets.

Control API

netif_napi_add() and netif_napi_del() add/remove a NAPI instance from the system. The instances are attached to the netdevice passed as argument (and will be deleted automatically when netdevice is unregistered). Instances are added in a disabled state.

napi_enable() and napi_disable() manage the disabled state. A disabled NAPI can’t be scheduled and its poll method is guaranteed to not be invoked. napi_disable() waits for ownership of the NAPI instance to be released.

The control APIs are not idempotent. Control API calls are safe against concurrent use of datapath APIs but an incorrect sequence of control API calls may result in crashes, deadlocks, or race conditions. For example, calling napi_disable() multiple times in a row will deadlock.

Datapath API

napi_schedule() is the basic method of scheduling a NAPI poll. Drivers should call this function in their interrupt handler (see Scheduling and IRQ masking for more info). A successful call to napi_schedule() will take ownership of the NAPI instance.

Later, after NAPI is scheduled, the driver’s poll method will be called to process the events/packets. The method takes a budget argument - drivers can process completions for any number of Tx packets but should only process up to budget number of Rx packets. Rx processing is usually much more expensive.

In other words for Rx processing the budget argument limits how many packets driver can process in a single poll. Rx specific APIs like page pool or XDP cannot be used at all when budget is 0. skb Tx processing should happen regardless of the budget, but if the argument is 0 driver cannot call any XDP (or page pool) APIs.

Warning

The budget argument may be 0 if core tries to only process skb Tx completions and no Rx or XDP packets.

The poll method returns the amount of work done. If the driver still has outstanding work to do (e.g. budget was exhausted) the poll method should return exactly budget. In that case, the NAPI instance will be serviced/polled again (without the need to be scheduled).

If event processing has been completed (all outstanding packets processed) the poll method should call napi_complete_done() before returning. napi_complete_done() releases the ownership of the instance.

Warning

The case of finishing all events and using exactly budget must be handled carefully. There is no way to report this (rare) condition to the stack, so the driver must either not call napi_complete_done() and wait to be called again, or return budget - 1.

If the budget is 0 napi_complete_done() should never be called.

Call sequence

Drivers should not make assumptions about the exact sequencing of calls. The poll method may be called without the driver scheduling the instance (unless the instance is disabled). Similarly, it’s not guaranteed that the poll method will be called, even if napi_schedule() succeeded (e.g. if the instance gets disabled).

As mentioned in the Control API section - napi_disable() and subsequent calls to the poll method only wait for the ownership of the instance to be released, not for the poll method to exit. This means that drivers should avoid accessing any data structures after calling napi_complete_done().

Scheduling and IRQ masking

Drivers should keep the interrupts masked after scheduling the NAPI instance - until NAPI polling finishes any further interrupts are unnecessary.

Drivers which have to mask the interrupts explicitly (as opposed to IRQ being auto-masked by the device) should use the napi_schedule_prep() and __napi_schedule() calls:

if (napi_schedule_prep(&v->napi)) {
    mydrv_mask_rxtx_irq(v->idx);
    /* schedule after masking to avoid races */
    __napi_schedule(&v->napi);
}

IRQ should only be unmasked after a successful call to napi_complete_done():

if (budget && napi_complete_done(&v->napi, work_done)) {
  mydrv_unmask_rxtx_irq(v->idx);
  return min(work_done, budget - 1);
}

napi_schedule_irqoff() is a variant of napi_schedule() which takes advantage of guarantees given by being invoked in IRQ context (no need to mask interrupts). napi_schedule_irqoff() will fall back to napi_schedule() if IRQs are threaded (such as if PREEMPT_RT is enabled).

Instance to queue mapping

Modern devices have multiple NAPI instances (struct napi_struct) per interface. There is no strong requirement on how the instances are mapped to queues and interrupts. NAPI is primarily a polling/processing abstraction without specific user-facing semantics. That said, most networking devices end up using NAPI in fairly similar ways.

NAPI instances most often correspond 1:1:1 to interrupts and queue pairs (queue pair is a set of a single Rx and single Tx queue).

In less common cases a NAPI instance may be used for multiple queues or Rx and Tx queues can be serviced by separate NAPI instances on a single core. Regardless of the queue assignment, however, there is usually still a 1:1 mapping between NAPI instances and interrupts.

It’s worth noting that the ethtool API uses a “channel” terminology where each channel can be either rx, tx or combined. It’s not clear what constitutes a channel; the recommended interpretation is to understand a channel as an IRQ/NAPI which services queues of a given type. For example, a configuration of 1 rx, 1 tx and 1 combined channel is expected to utilize 3 interrupts, 2 Rx and 2 Tx queues.

User API

User interactions with NAPI depend on NAPI instance ID. The instance IDs are only visible to the user thru the SO_INCOMING_NAPI_ID socket option. It’s not currently possible to query IDs used by a given device.

Software IRQ coalescing

NAPI does not perform any explicit event coalescing by default. In most scenarios batching happens due to IRQ coalescing which is done by the device. There are cases where software coalescing is helpful.

NAPI can be configured to arm a repoll timer instead of unmasking the hardware interrupts as soon as all packets are processed. The gro_flush_timeout sysfs configuration of the netdevice is reused to control the delay of the timer, while napi_defer_hard_irqs controls the number of consecutive empty polls before NAPI gives up and goes back to using hardware IRQs.

Busy polling

Busy polling allows a user process to check for incoming packets before the device interrupt fires. As is the case with any busy polling it trades off CPU cycles for lower latency (production uses of NAPI busy polling are not well known).

Busy polling is enabled by either setting SO_BUSY_POLL on selected sockets or using the global net.core.busy_poll and net.core.busy_read sysctls. An io_uring API for NAPI busy polling also exists.

IRQ mitigation

While busy polling is supposed to be used by low latency applications, a similar mechanism can be used for IRQ mitigation.

Very high request-per-second applications (especially routing/forwarding applications and especially applications using AF_XDP sockets) may not want to be interrupted until they finish processing a request or a batch of packets.

Such applications can pledge to the kernel that they will perform a busy polling operation periodically, and the driver should keep the device IRQs permanently masked. This mode is enabled by using the SO_PREFER_BUSY_POLL socket option. To avoid system misbehavior the pledge is revoked if gro_flush_timeout passes without any busy poll call.

The NAPI budget for busy polling is lower than the default (which makes sense given the low latency intention of normal busy polling). This is not the case with IRQ mitigation, however, so the budget can be adjusted with the SO_BUSY_POLL_BUDGET socket option.

Threaded NAPI

Threaded NAPI is an operating mode that uses dedicated kernel threads rather than software IRQ context for NAPI processing. The configuration is per netdevice and will affect all NAPI instances of that device. Each NAPI instance will spawn a separate thread (called napi/${ifc-name}-${napi-id}).

It is recommended to pin each kernel thread to a single CPU, the same CPU as the CPU which services the interrupt. Note that the mapping between IRQs and NAPI instances may not be trivial (and is driver dependent). The NAPI instance IDs will be assigned in the opposite order than the process IDs of the kernel threads.

Threaded NAPI is controlled by writing 0/1 to the threaded file in netdev’s sysfs directory.

Footnotes

[1]

NAPI was originally referred to as New API in 2.4 Linux.