Vernon Mauery [off-list ref] writes:

So while this issue really hits -rt kernels hard, it has a real effect on
mainline kernels as well.  The contention of the spinlocks is amplified
when they get turned into rt-mutexes, which causes a double context switch.

The new adaptive spin heuristics that have been discussed some time
ago didn't help? Unsurprisingly making locks a lot more expensive
has drawbacks as you discovered.

   &list->lock#3:      24517307       24643791           0.71        1286.62      56516392.42       34834296       44904018           0.60        164.79    31314786.02
    -------------
   &list->lock#3       15596927    [<ffffffff812474da>] dev_queue_xmit+0x2ea/0x468

The real "fix" would be probably to use a multi queue capable NIC
and a NIC driver that sets up multiple queues for TX (normally they
only do for RX). Then cores or a set of cores (often the number
of cores is larger than the number of NIC queues) could avoid this
problem. Disadvantage: more memory use.

But then again I'm not sure it's  worth it if the problem only
happens in out of tree RT.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.