How to add a new qdisc in linux - linux-kernel

Question

I am trying to modify the Red Algorithm (http://en.wikipedia.org/wiki/Random_early_detection) for certain experiments.
After modifying the code, I loaded onto the kernel using the insmod command.
I verified the successful loading by using lsmod | grep red_new
However when I try to use the tc qdisc command it fails giving the following error:
tc qdisc add dev eth0 root red_new limit 100 min 80 max 90 avpkt 10 burst 10 probability 1 bandwidth 200 ecn
unknown qdisc "red_new" hence option "limit" is unparsable
What could be the possible reason ?
After running the ltrace command suggested by ymonad I get the following output:
strlen("red_new") = 7
strlen("red_new") = 7
strlen("red_new") = 7
strncpy(0x7fff6467ad10, "red_new", 15) = 0x7fff6467ad10
dlopen("./tc/q_red_new.so", 1) = 0x1abe030
dlsym(0x1abe030, "red_new_qdisc_util") = 0x7f62bdd240c0
memcpy(0x7fff6467ad48, "red_new\0", 8) = 0x7fff6467ad48
I ran the tc qdisc show to check if it was added but it hasn't.
tc qdisc show
qdisc mq 0: dev eth0 root
qdisc mq 0: dev eth1 root
qdisc mq 0: dev eth2 root
qdisc mq 0: dev eth3 root

Answer 1

According to the result of strace tc qdisc add dev eth0 root red_new, and source of tc command, it seems that tc is searching for $TC_LIB_DIR/q_red_new.so.
You have to create the module for your own. I would give you small instruction.
(1) Download source of iproute2 from following url, extract it, and cd to the folder.
https://wiki.linuxfoundation.org/networking/iproute2
(2) Copy q_red.c to q_red_new.c
$ cp tc/q_red.c tc/q_red_new.c
(3) Edit tc/q_red_new.c
Rename red_parse_opt, red_print_opt, red_print_xstats, to red_new_parse and so on.
Additionally you have to rename red_qdisk_util to req_new_qdisc_util and change the id and other members.
struct qdisc_util red_new_qdisc_util = {
.id = "red_new",
.parse_qopt = red_new_parse_opt,
.print_qopt = red_new_print_opt,
.print_xstats = red_new_print_xstats,
};
(4) Configure and build q_red_new.so
$ ./configure
$ make TCSO=q_red_new.so
now you see that ./tc/q_red_new.so is created
(5) Run tc command with TC_LIB_DIR environment.
$ TC_LIB_DIR='./tc' tc qdisc add dev eth0 root red_new
UPDATE: here's how to know that the tc command loaded the q_red_new.so correctly.
if dlopen returns zero then you failed to load./tc/q_red_new.so.
if dlsym returns zero then you failed to load red_new_qdisc_util inside the q_red_new.so.
# export TC_LIB_DIR='./tc'
# ltrace ./tc/tc qdisc add dev eth0 root red_new limit 100 min 80 max 90 avpkt 10 burst 10 probability 1 bandwidth 200 ecn 2>&1 | grep red_new
.. OMITTED ..
dlopen("./tc/q_red_new.so", 1) = 0x12c1030
snprintf("red_new_qdisc_util", 256, "%s_qdisc_util", "red_new") = 18
dlsym(0x12c1030, "red_new_qdisc_util") = 0x7f1cf0d6cc40
.. OMITTED ..