linux/net
David Woodhouse ae088d663b atm: br2684: Fix excessive queue bloat
There's really no excuse for an additional wmem_default of buffering
between the netdev queue and the ATM device. Two packets (one in-flight,
and one ready to send) ought to be fine. It's not as if it should take
long to get another from the netdev queue when we need it.

If necessary we can make the queue space configurable later, but I don't
think it's likely to be necessary.

cf. commit 9d02daf754 (pppoatm: Fix
excessive queue bloat) which did something very similar for PPPoATM.

Note that there is a tremendously unlikely race condition which may
result in qspace temporarily going negative. If a CPU running the
br2684_pop() function goes off into the weeds for a long period of time
after incrementing qspace to 1, but before calling netdev_wake_queue()...
and another CPU ends up calling br2684_start_xmit() and *stopping* the
queue again before the first CPU comes back, the netdev queue could
end up being woken when qspace has already reached zero.

An alternative approach to coping with this race would be to check in
br2684_start_xmit() for qspace==0 and return NETDEV_TX_BUSY, but just
using '> 0' and '< 1' for comparison instead of '== 0' and '!= 0' is
simpler. It just warranted a mention of *why* we do it that way...

Move the call to atmvcc->send() to happen *after* the accounting and
potentially stopping the netdev queue, in br2684_xmit_vcc(). This matters
if the ->send() call suffers an immediate failure, because it'll call
br2684_pop() with the offending skb before returning. We want that to
happen *after* we've done the initial accounting for the packet in
question. Also make it return an appropriate success/failure indication
while we're at it.

Tested by running 'ping -l 1000 bottomless.aaisp.net.uk' from within my
network, with only a single PPPoE-over-BR2684 link running. And after
setting txqueuelen on the nas0 interface to something low (5, in fact).
Before the patch, we'd see about 15 packets being queued and a resulting
latency of ~56ms being reached. After the patch, we see only about 8,
which is fairly much what we expect. And a max latency of ~36ms. On this
OpenWRT box, wmem_default is 163840.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Reviewed-by: Krzysztof Mazur <krzysiek@podlesie.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
2012-11-26 17:13:56 -05:00
..
9p The following changes since commit 4cbe5a555f: 2012-10-12 09:59:23 +09:00
802
8021q net: Allow the userns root to control vlans. 2012-11-18 20:33:00 -05:00
appletalk userns: Print out socket uids in a user namespace aware fashion. 2012-08-14 21:48:06 -07:00
atm atm: br2684: Fix excessive queue bloat 2012-11-26 17:13:56 -05:00
ax25 userns: Convert net/ax25 to use kuid_t where appropriate 2012-08-14 21:49:42 -07:00
batman-adv batman-adv: Use packing of 2 for all headers before an ethernet header 2012-11-21 12:35:47 +01:00
bluetooth Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2012-11-17 22:00:43 -05:00
bridge net: Enable a userns root rtnl calls that are safe for unprivilged users 2012-11-18 20:33:36 -05:00
caif caif: Remove redundant null check before kfree in cfctrl.c 2012-11-20 13:48:09 -05:00
can net: Push capable(CAP_NET_ADMIN) into the rtnl methods 2012-11-18 20:32:44 -05:00
ceph Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/sage/ceph-client 2012-10-29 08:49:25 -07:00
core Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2012-11-25 12:49:17 -05:00
dcb net: Push capable(CAP_NET_ADMIN) into the rtnl methods 2012-11-18 20:32:44 -05:00
dccp tcp: better retrans tracking for defer-accept 2012-11-03 14:45:00 -04:00
decnet net: Push capable(CAP_NET_ADMIN) into the rtnl methods 2012-11-18 20:32:44 -05:00
dns_resolver Merge branch 'modules-next' of git://git.kernel.org/pub/scm/linux/kernel/git/rusty/linux 2012-10-14 13:39:34 -07:00
dsa dsa: Hide core config options; make drivers select what they need 2012-11-26 17:10:44 -05:00
ethernet ipx: move peII functions 2012-07-19 10:48:00 -07:00
ieee802154 net/ieee802154/6lowpan.c: Remove unecessary semicolon 2012-09-18 16:08:19 -04:00
ipv4 ipv4/ipmr and ipv6/ip6mr: Convert int mroute_do_<foo> to bool 2012-11-25 16:34:17 -05:00
ipv6 ipv4/ipmr and ipv6/ip6mr: Convert int mroute_do_<foo> to bool 2012-11-25 16:34:17 -05:00
ipx userns: Print out socket uids in a user namespace aware fashion. 2012-08-14 21:48:06 -07:00
irda Merge 3.7-rc1 into tty-linus 2012-10-14 22:41:27 -07:00
iucv net: remove skb_orphan_try() 2012-06-15 15:30:15 -07:00
key net: Allow userns root to control llc, netfilter, netlink, packet, and xfrm 2012-11-18 20:32:45 -05:00
l2tp Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2012-11-10 18:32:51 -05:00
lapb
llc net: Allow userns root to control llc, netfilter, netlink, packet, and xfrm 2012-11-18 20:32:45 -05:00
mac80211 Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2012-11-25 12:49:17 -05:00
mac802154 mac802154: sparse warnings: make symbols static 2012-07-12 07:54:45 -07:00
netfilter Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2012-11-25 12:49:17 -05:00
netlabel Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next 2012-10-02 13:38:27 -07:00
netlink net: Allow userns root to control llc, netfilter, netlink, packet, and xfrm 2012-11-18 20:32:45 -05:00
netrom net: change return values from -EACCES to -EPERM 2012-09-21 13:58:08 -04:00
nfc Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2012-11-25 12:49:17 -05:00
openvswitch net/openvswitch/vport.c: Remove unecessary semicolon 2012-09-18 16:08:19 -04:00
packet net: Allow userns root to control llc, netfilter, netlink, packet, and xfrm 2012-11-18 20:32:45 -05:00
phonet net: Push capable(CAP_NET_ADMIN) into the rtnl methods 2012-11-18 20:32:44 -05:00
rds net: rds: use this_cpu_* per-cpu helper 2012-11-19 18:59:44 -05:00
rfkill Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next 2012-10-02 13:38:27 -07:00
rose
rxrpc Merge branch 'modules-next' of git://git.kernel.org/pub/scm/linux/kernel/git/rusty/linux 2012-10-14 13:39:34 -07:00
sched net: sched: enable CAN Identifier to be build into kernel 2012-11-25 16:06:06 -05:00
sctp sctp: send abort chunk when max_retrans exceeded 2012-11-20 15:50:37 -05:00
sunrpc SUNRPC: return proper errno from backchannel_rqst 2012-11-01 11:50:53 -04:00
tipc tipc: delete TIPC_ADVANCED Kconfig variable 2012-11-22 14:33:29 -05:00
unix net: Don't export sysctls to unprivileged users 2012-11-18 20:30:55 -05:00
wanrouter wanmain: comparing array with NULL 2012-07-24 13:55:21 -07:00
wimax
wireless Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2012-11-17 22:00:43 -05:00
x25 net: Fix (nearly-)kernel-doc comments for various functions 2012-07-10 23:13:45 -07:00
xfrm Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/klassert/ipsec-next 2012-11-22 15:25:55 -05:00
compat.c make get_file() return its argument 2012-09-26 21:10:25 -04:00
Kconfig net: Add INET dependency on aes crypto for the sake of TCP fastopen. 2012-09-04 14:20:14 -04:00
Makefile ipv6: Preserve ipv6 functionality needed by NET 2012-11-18 02:34:00 -05:00
nonet.c
socket.c cgroup: net_cls: Rework update socket logic 2012-10-26 03:40:51 -04:00
sysctl_net.c user_ns: get rid of duplicate code in net_ctl_permissions 2012-11-18 20:32:45 -05:00