From Flavio Leitner:
On many core SMP machine (such as 16 core or more), soft lockup can occur when heavy network load are produced concurrently.

The lockup happens at __qdisc_run()(@net/sched/sch_generic.c:line 84). Because driver continue to send packet and return NETDEV_TX_OK, __qdisc_run() can't exit from qdisc_restart() loop.

This behavior may improve throughput, but some application can stuck over 10s.
This issue has been fixed on vanilla kernel.

Version-Release number of selected component:
kernel version: 2.6.18-92.el5 (RHEL5.2GA)

How reproducible:
It can be reproducible in dozens of seconds, on 16 core SMP box. This issue is easy to happen, when UDP workload is very high.

Steps to Reproduce:
On 16 core SMP machine, execute netperf in higher than 16 parallel with the following options, then it occurs at a client side.

# netperf -H <netserver_address> -l 60 -t UDP_STREAM -- -s 262144 -r 262144 -m
16384

Actual results:
A lot of soft lockup messages are recorded into syslog, and performance problem appears in some applications.

Expected results:
In kernel, any CPU doesn't dedicate to some work without schedule() for a long time.

Hardware info:
Express5800/140Rf-4

Business impact:
It makes customer's applications unresponsive too long and it makes impossible to apply RHEL5.2 to performance/latency sensitive systems.

Additional info:
git patch: [NET]: Add preemption point in qdisc_run
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=2ba2506ca7ca62c56edaa334b0fe61eb5eab6ab0