| ALTQ(9) | Kernel Developer's Manual | ALTQ(9) |
NAME¶
ALTQ — kernel
interfaces for manipulating output queues on network interfaces
SYNOPSIS¶
#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, or
ENOBUFS 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 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;
}; | /* driver queue fields */
| ......
| /* altq related fields */
| ......
| };
|
##old-style## ##new-style##
|
struct ifnet { | struct ifnet {
.... | ....
|
struct ifqueue if_snd; | struct ifaltq if_snd;
|
.... | ....
}; | };
|
IFQ_*() macros look like:
#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)
IFQ_ENQUEUE() does the following:
- queue a packet,
- drop (and free) a packet if the enqueue operation fails.
If the enqueue operation fails, error is set
to 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))
|
IFQ_IS_EMPTY() only checks if there is any packet stored
in the queue. Note that even when IFQ_IS_EMPTY() is
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 is NULL 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;
|
if_start() from
transmission complete interrupts in order to trigger the next dequeue.
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 */
} | }
|
IFQ_DEQUEUE_NOLOCK() under the
same lock as a previous IFQ_POLL_NOLOCK() returns the
same packet. Note that they need to be guarded by
IFQ_LOCK().
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))
|
IFQ_DRV_DEQUEUE(),
IFQ_DRV_PREPEND() and
IFQ_DRV_PURGE() are protected with a mutex of some
kind.
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¶
HISTORY¶
The ALTQ system first appeared in March
1997 and found home in the KAME project (http://www.kame.net). It was
imported to FreeBSD in 5.3 .
| March 20, 2018 | Debian |