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ALTQ(9) | Kernel Developer's Manual | ALTQ(9) |
NAME¶
ALTQ — kernel interfaces for manipulating output queues on network interfacesSYNOPSIS¶
#include <sys/types.h>#include <sys/socket.h>
#include <net/if.h>
#include <net/if_var.h>
Enqueue macros¶
IFQ_ENQUEUE(struct ifaltq *ifq, struct mbuf *m, int error); IFQ_HANDOFF(struct ifnet *ifp, struct mbuf *m, int error); IFQ_HANDOFF_ADJ(struct ifnet *ifp, struct mbuf *m, int adjust, int error);Dequeue macros¶
IFQ_DEQUEUE(struct ifaltq *ifq, struct mbuf *m); IFQ_POLL_NOLOCK(struct ifaltq *ifq, struct mbuf *m); IFQ_PURGE(struct ifaltq *ifq); IFQ_IS_EMPTY(struct ifaltq *ifq);Driver managed dequeue macros¶
IFQ_DRV_DEQUEUE(struct ifaltq *ifq, struct mbuf *m); IFQ_DRV_PREPEND(struct ifaltq *ifq, struct mbuf *m); IFQ_DRV_PURGE(struct ifaltq *ifq); IFQ_DRV_IS_EMPTY(struct ifaltq *ifq);General setup macros¶
IFQ_SET_MAXLEN(struct ifaltq *ifq, int len); IFQ_INC_LEN(struct ifaltq *ifq); IFQ_DEC_LEN(struct ifaltq *ifq); IFQ_INC_DROPS(struct ifaltq *ifq); IFQ_SET_READY(struct ifaltq *ifq);DESCRIPTION¶
The ALTQ system is a framework to manage queuing disciplines on network interfaces. ALTQ introduces new macros to manipulate output queues. The output queue macros are used to abstract queue operations and not to touch the internal fields of the output queue structure. The macros are independent from the ALTQ implementation, and compatible with the traditional ifqueue macros for ease of transition. IFQ_ENQUEUE(), IFQ_HANDOFF() and IFQ_HANDOFF_ADJ() enqueue a packet m to the queue ifq. The underlying queuing discipline may discard the packet. The error argument is set to 0 on success, orENOBUFS
if the packet is discarded. The
packet pointed to by m will be freed by the device
driver on success, or by the queuing discipline on failure, so the caller
should not touch m after enqueuing.
IFQ_HANDOFF() and IFQ_HANDOFF_ADJ()
combine the enqueue operation with statistic generation and call
if_start() upon successful enqueue to initiate the actual
send.
IFQ_DEQUEUE() dequeues a packet from the queue. The dequeued
packet is returned in m, or m is
set to NULL
if no packet is dequeued. The caller must
always check m since a non-empty queue could return
NULL
under rate-limiting.
IFQ_POLL_NOLOCK() returns the next packet without removing it
from the queue. The caller must hold the queue mutex when calling
IFQ_POLL_NOLOCK() in order to guarantee that a subsequent
call to IFQ_DEQUEUE_NOLOCK() dequeues the same packet.
IFQ_*_NOLOCK() variants (if available) always assume that the
caller holds the queue mutex. They can be grabbed with
IFQ_LOCK() and released with IFQ_UNLOCK().
IFQ_PURGE() discards all the packets in the queue. The purge
operation is needed since a non-work conserving queue cannot be emptied by a
dequeue loop.
IFQ_IS_EMPTY() can be used to check if the queue is empty.
Note that IFQ_DEQUEUE() could still return
NULL
if the queuing discipline is non-work conserving.
IFQ_DRV_DEQUEUE() moves up to
ifq->ifq_drv_maxlen packets from the queue to the
“driver managed” queue and returns the first one via
m. As for IFQ_DEQUEUE(),
m can be NULL
even for a
non-empty queue. Subsequent calls to IFQ_DRV_DEQUEUE() pass
the packets from the “driver managed” queue without obtaining the
queue mutex. It is the responsibility of the caller to protect against
concurrent access. Enabling ALTQ for a given queue sets
ifq_drv_maxlen to 0 as the “bulk dequeue”
performed by IFQ_DRV_DEQUEUE() for higher values of
ifq_drv_maxlen is adverse to ALTQ's
internal timing. Note that a driver must not mix IFQ_DRV_*()
macros with the default dequeue macros as the default macros do not look at
the “driver managed” queue which might lead to an mbuf leak.
IFQ_DRV_PREPEND() prepends m to the
“driver managed” queue from where it will be obtained with the
next call to IFQ_DRV_DEQUEUE().
IFQ_DRV_PURGE() flushes all packets in the “driver
managed” queue and calls to IFQ_PURGE() afterwards.
IFQ_DRV_IS_EMPTY() checks for packets in the “driver
managed” part of the queue. If it is empty, it forwards to
IFQ_IS_EMPTY().
IFQ_SET_MAXLEN() sets the queue length limit to the default
FIFO queue. The ifq_drv_maxlen member of the
ifaltq structure controls the length limit of the
“driver managed” queue.
IFQ_INC_LEN() and IFQ_DEC_LEN() increment or
decrement the current queue length in packets. This is mostly for internal
purposes.
IFQ_INC_DROPS() increments the drop counter and is identical
to IF_DROP(). It is defined for naming consistency only.
IFQ_SET_READY() sets a flag to indicate that a driver was
converted to use the new macros. ALTQ can be enabled only on
interfaces with this flag.
COMPATIBILITY¶
ifaltq Ss structure¶
In order to keep compatibility with the existing code, the new output queue structure ifaltq has the same fields. The traditional IF_*() macros and the code directly referencing the fields within if_snd still work with ifaltq.##old-style## ##new-style## | struct ifqueue { | struct ifaltq { struct mbuf *ifq_head; | struct mbuf *ifq_head; struct mbuf *ifq_tail; | struct mbuf *ifq_tail; int ifq_len; | int ifq_len; int ifq_maxlen; | int ifq_maxlen; int ifq_drops; | int ifq_drops; }; | /* driver queue fields */ | ...... | /* altq related fields */ | ...... | }; |
##old-style## ##new-style## | struct ifnet { | struct ifnet { .... | .... | struct ifqueue if_snd; | struct ifaltq if_snd; | .... | .... }; | }; |
#define IFQ_DEQUEUE(ifq, m) \ if (ALTQ_IS_ENABLED((ifq)) \ ALTQ_DEQUEUE((ifq), (m)); \ else \ IF_DEQUEUE((ifq), (m));
Enqueue operation¶
The semantics of the enqueue operation is changed. In the new style, enqueue and packet drop are combined since they cannot be easily separated in many queuing disciplines. The new enqueue operation corresponds to the following macro that is written with the old macros.#define IFQ_ENQUEUE(ifq, m, error) \ do { \ if (IF_QFULL((ifq))) { \ m_freem((m)); \ (error) = ENOBUFS; \ IF_DROP(ifq); \ } else { \ IF_ENQUEUE((ifq), (m)); \ (error) = 0; \ } \ } while (0)
- queue a packet,
- drop (and free) a packet if the enqueue operation fails.
ENOBUFS
. The m mbuf is freed by
the queuing discipline. The caller should not touch mbuf after calling
IFQ_ENQUEUE() so that the caller may need to copy
m_pkthdr.len or m_flags field
beforehand for statistics. IFQ_HANDOFF() and
IFQ_HANDOFF_ADJ() can be used if only default interface
statistics and an immediate call to if_start() are desired.
The caller should not use senderr() since mbuf was already
freed.
The new style if_output() looks as follows:
##old-style## ##new-style## | int | int ether_output(ifp, m0, dst, rt0) | ether_output(ifp, m0, dst, rt0) { | { ...... | ...... | | mflags = m->m_flags; | len = m->m_pkthdr.len; s = splimp(); | s = splimp(); if (IF_QFULL(&ifp->if_snd)) { | IFQ_ENQUEUE(&ifp->if_snd, m, | error); IF_DROP(&ifp->if_snd); | if (error != 0) { splx(s); | splx(s); senderr(ENOBUFS); | return (error); } | } IF_ENQUEUE(&ifp->if_snd, m); | ifp->if_obytes += | ifp->if_obytes += len; m->m_pkthdr.len; | if (m->m_flags & M_MCAST) | if (mflags & M_MCAST) ifp->if_omcasts++; | ifp->if_omcasts++; | if ((ifp->if_flags & IFF_OACTIVE) | if ((ifp->if_flags & IFF_OACTIVE) == 0) | == 0) (*ifp->if_start)(ifp); | (*ifp->if_start)(ifp); splx(s); | splx(s); return (error); | return (error); | bad: | bad: if (m) | if (m) m_freem(m); | m_freem(m); return (error); | return (error); } | } |
HOW TO CONVERT THE EXISTING DRIVERS¶
First, make sure the corresponding if_output() is already converted to the new style. Look for if_snd in the driver. Probably, you need to make changes to the lines that include if_snd.Empty check operation¶
If the code checks ifq_head to see whether the queue is empty or not, use IFQ_IS_EMPTY().##old-style## ##new-style## | if (ifp->if_snd.ifq_head != NULL) | if (!IFQ_IS_EMPTY(&ifp->if_snd)) |
FALSE
, IFQ_DEQUEUE() could still
return NULL
if the queue is under rate-limiting.
Dequeue operation¶
Replace IF_DEQUEUE() by IFQ_DEQUEUE(). Always check whether the dequeued mbuf isNULL
or not.
Note that even when IFQ_IS_EMPTY() is
FALSE
, IFQ_DEQUEUE() could return
NULL
due to rate-limiting.
##old-style## ##new-style## | IF_DEQUEUE(&ifp->if_snd, m); | IFQ_DEQUEUE(&ifp->if_snd, m); | if (m == NULL) | return; |
Poll-and-dequeue operation¶
If the code polls the packet at the head of the queue and actually uses the packet before dequeuing it, use IFQ_POLL_NOLOCK() and IFQ_DEQUEUE_NOLOCK().##old-style## ##new-style## | | IFQ_LOCK(&ifp->if_snd); m = ifp->if_snd.ifq_head; | IFQ_POLL_NOLOCK(&ifp->if_snd, m); if (m != NULL) { | if (m != NULL) { | /* use m to get resources */ | /* use m to get resources */ if (something goes wrong) | if (something goes wrong) | IFQ_UNLOCK(&ifp->if_snd); return; | return; | IF_DEQUEUE(&ifp->if_snd, m); | IFQ_DEQUEUE_NOLOCK(&ifp->if_snd, m); | IFQ_UNLOCK(&ifp->if_snd); | /* kick the hardware */ | /* kick the hardware */ } | } |
Eliminating IF_PREPEND()¶
If the code uses IF_PREPEND(), you have to eliminate it unless you can use a “driver managed” queue which allows the use of IFQ_DRV_PREPEND() as a substitute. A common usage of IF_PREPEND() is to cancel the previous dequeue operation. You have to convert the logic into poll-and-dequeue.##old-style## ##new-style## | | IFQ_LOCK(&ifp->if_snd); IF_DEQUEUE(&ifp->if_snd, m); | IFQ_POLL_NOLOCK(&ifp->if_snd, m); if (m != NULL) { | if (m != NULL) { | if (something_goes_wrong) { | if (something_goes_wrong) { IF_PREPEND(&ifp->if_snd, m); | IFQ_UNLOCK(&ifp->if_snd); return; | return; } | } | | /* at this point, the driver | * is committed to send this | * packet. | */ | IFQ_DEQUEUE_NOLOCK(&ifp->if_snd, m); | IFQ_UNLOCK(&ifp->if_snd); | /* kick the hardware */ | /* kick the hardware */ } | } |
Purge operation¶
Use IFQ_PURGE() to empty the queue. Note that a non-work conserving queue cannot be emptied by a dequeue loop.##old-style## ##new-style## | while (ifp->if_snd.ifq_head != NULL) {| IFQ_PURGE(&ifp->if_snd); IF_DEQUEUE(&ifp->if_snd, m); | m_freem(m); | } | |
Conversion using a driver managed queue¶
Convert IF_*() macros to their equivalent IFQ_DRV_*() and employ IFQ_DRV_IS_EMPTY() where appropriate.##old-style## ##new-style## | if (ifp->if_snd.ifq_head != NULL) | if (!IFQ_DRV_IS_EMPTY(&ifp->if_snd)) |
Attach routine¶
Use IFQ_SET_MAXLEN() to set ifq_maxlen to len. Initialize ifq_drv_maxlen with a sensible value if you plan to use the IFQ_DRV_*() macros. Add IFQ_SET_READY() to show this driver is converted to the new style. (This is used to distinguish new-style drivers.)##old-style## ##new-style## | ifp->if_snd.ifq_maxlen = qsize; | IFQ_SET_MAXLEN(&ifp->if_snd, qsize); | ifp->if_snd.ifq_drv_maxlen = qsize; | IFQ_SET_READY(&ifp->if_snd); if_attach(ifp); | if_attach(ifp); |
Other issues¶
The new macros for statistics:##old-style## ##new-style## | IF_DROP(&ifp->if_snd); | IFQ_INC_DROPS(&ifp->if_snd); | ifp->if_snd.ifq_len++; | IFQ_INC_LEN(&ifp->if_snd); | ifp->if_snd.ifq_len--; | IFQ_DEC_LEN(&ifp->if_snd); |
QUEUING DISCIPLINES¶
Queuing disciplines need to maintain ifq_len (used by IFQ_IS_EMPTY()). Queuing disciplines also need to guarantee that the same mbuf is returned if IFQ_DEQUEUE() is called immediately after IFQ_POLL().SEE ALSO¶
pf(4), pf.conf(5), pfctl(8)HISTORY¶
The ALTQ system first appeared in March 1997.August 25, 2004 | Debian |