codel: Controlled Delay AQM

An implementation of CoDel AQM, from Kathleen Nichols and Van Jacobson.

http://queue.acm.org/detail.cfm?id=2209336

This AQM main input is no longer queue size in bytes or packets, but the
delay packets stay in (FIFO) queue.

As we don't have infinite memory, we still can drop packets in enqueue()
in case of massive load, but mean of CoDel is to drop packets in
dequeue(), using a control law based on two simple parameters :

target : target sojourn time (default 5ms)
interval : width of moving time window (default 100ms)

Based on initial work from Dave Taht.

Refactored to help future codel inclusion as a plugin for other linux
qdisc (FQ_CODEL, ...), like RED.

include/net/codel.h contains codel algorithm as close as possible than
Kathleen reference.

net/sched/sch_codel.c contains the linux qdisc specific glue.

Separate structures permit a memory efficient implementation of fq_codel
(to be sent as a separate work) : Each flow has its own struct
codel_vars.

timestamps are taken at enqueue() time with 1024 ns precision, allowing
a range of 2199 seconds in queue, and 100Gb links support. iproute2 uses
usec as base unit.

Selected packets are dropped, unless ECN is enabled and packets can get
ECN mark instead.

Tested from 2Mb to 10Gb speeds with no particular problems, on ixgbe and
tg3 drivers (BQL enabled).

Usage: tc qdisc ... codel [ limit PACKETS ] [ target TIME ]
                          [ interval TIME ] [ ecn ]

qdisc codel 10: parent 1:1 limit 2000p target 3.0ms interval 60.0ms ecn
 Sent 13347099587 bytes 8815805 pkt (dropped 0, overlimits 0 requeues 0)
 rate 202365Kbit 16708pps backlog 113550b 75p requeues 0
  count 116 lastcount 98 ldelay 4.3ms dropping drop_next 816us
  maxpacket 1514 ecn_mark 84399 drop_overlimit 0

CoDel must be seen as a base module, and should be used keeping in mind
there is still a FIFO queue. So a typical setup will probably need a
hierarchy of several qdiscs and packet classifiers to be able to meet
whatever constraints a user might have.

One possible example would be to use fq_codel, which combines Fair
Queueing and CoDel, in replacement of sfq / sfq_red.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Dave Taht <dave.taht@bufferbloat.net>
Cc: Kathleen Nichols <nichols@pollere.com>
Cc: Van Jacobson <van@pollere.net>
Cc: Tom Herbert <therbert@google.com>
Cc: Matt Mathis <mattmathis@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 26 insertions, 0 deletions

diff --git a/include/linux/pkt_sched.h b/include/linux/pkt_sched.h
index ffe975c3f1d8..cde56c22bdab 100644
--- a/include/linux/pkt_sched.h
+++ b/include/linux/pkt_sched.h
@@ -655,4 +655,30 @@ struct tc_qfq_stats {
         __u32 lmax;
 };
 
+/* CODEL */
+
+enum {
+        TCA_CODEL_UNSPEC,
+        TCA_CODEL_TARGET,
+        TCA_CODEL_LIMIT,
+        TCA_CODEL_INTERVAL,
+        TCA_CODEL_ECN,
+        __TCA_CODEL_MAX
+};
+
+#define TCA_CODEL_MAX   (__TCA_CODEL_MAX - 1)
+
+struct tc_codel_xstats {
+        __u32   maxpacket; /* largest packet we've seen so far */
+        __u32   count;     /* how many drops we've done since the last time we
+                            * entered dropping state
+                            */
+        __u32   lastcount; /* count at entry to dropping state */
+        __u32   ldelay;    /* in-queue delay seen by most recently dequeued packet */
+        __s32   drop_next; /* time to drop next packet */
+        __u32   drop_overlimit; /* number of time max qdisc packet limit was hit */
+        __u32   ecn_mark;  /* number of packets we ECN marked instead of dropped */
+        __u32   dropping;  /* are we in dropping state ? */
+};
+
 #endif