On Wed, Mar 15, 2023 at 03:30:44PM -0700, Jakub Kicinski wrote: > -See https://wiki.linuxfoundation.org/networking/napi for more > -information on NAPI. > +See :ref:`Documentation/networking/napi.rst <napi>` for more information. > <snipped>... > -For more information on NAPI, see > -https://wiki.linuxfoundation.org/networking/napi > + > +See :ref:`Documentation/networking/napi.rst <napi>` for more information. > > <snipped>... > -See https://wiki.linuxfoundation.org/networking/napi for more information on > -NAPI. > - > +See :ref:`Documentation/networking/napi.rst <napi>` for more information. I prefer not to use :ref:, but simply write out the full document path to achieve the same internal link. > +==== > +NAPI > +==== > + > +NAPI is the event handling mechanism used by the Linux networking stack. > +The name NAPI does not stand for anything in particular. > + > +In basic operation device notifies the host about new events via an interrupt. > +The host then schedules a NAPI instance to process the events. > +Device may also be polled for events via NAPI without receiving > +an interrupts first (busy polling). > + > +NAPI processing usually happens in the software interrupt context, > +but user may choose 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 which had been > +transmitted and process newly received packets. > + > +.. _drv_ctrl: > + > +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. > + > +Datapath API > +------------ > + > +napi_schedule() is the basic method of scheduling a NAPI poll. > +Drivers should call this function in their interrupt handler > +(see :ref:`drv_sched` for more info). Successful call to napi_schedule() > +will take ownership of the NAPI instance. > + > +Some time after NAPI is scheduled 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. > + > +.. warning:: > + > + ``budget`` may be 0 if core tries to only process Tx completions > + and no Rx packets. > + > +The poll method returns amount of work performed. If 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 ``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 driver scheduling > +the instance (unless the instance is disabled). Similarly if > +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 :ref:`drv_ctrl` 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(). > + > +.. _drv_sched: > + > +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: > + > +.. code-block:: c > + > + 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 successful call to napi_complete_done(): > + > +.. code-block:: c > + > + 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). Note that PREEMPT_RT forces all interrupts > +to be threaded so the interrupt may need to be marked ``IRQF_NO_THREAD`` > +to avoid issues on real-time kernel configurations. > + > +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 many user-facing semantics. That said, most networking > +devices end up using NAPI is 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 user process to check for incoming packets before > +device interrupt fires. As is the case with any busy polling it trades > +off CPU cycles for lower latency (in fact production uses of NAPI busy > +polling are not well known). > + > +User can enable busy polling 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 the 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 > +------------- > + > +Use 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 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. > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h > index 470085b121d3..b439f877bc3a 100644 > --- a/include/linux/netdevice.h > +++ b/include/linux/netdevice.h > @@ -508,15 +508,18 @@ static inline bool napi_reschedule(struct napi_struct *napi) > return false; > } > > -bool napi_complete_done(struct napi_struct *n, int work_done); > /** > - * napi_complete - NAPI processing complete > - * @n: NAPI context > + * napi_complete_done - NAPI processing complete > + * @n: NAPI context > + * @work_done: number of packets processed > * > - * Mark NAPI processing as complete. > - * Consider using napi_complete_done() instead. > + * Mark NAPI processing as complete. Should only be called if poll budget > + * has not been completely consumed. > + * Prefer over napi_complete(). > * Return false if device should avoid rearming interrupts. > */ > +bool napi_complete_done(struct napi_struct *n, int work_done); > + > static inline bool napi_complete(struct napi_struct *n) > { > return napi_complete_done(n, 0); The doc LGTM, thanks! Reviewed-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx> -- An old man doll... just what I always wanted! - Clara
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