From: Peter Hurley <redacted>
Date: Sat, 27 Feb 2016 12:29:39 -0800

Not really. softirq raised from interrupt context will always execute
on this cpu and not in ksoftirqd, unless load forces softirq loop abort.

That guarantee never was specified.

Or are you saying that by design, on a system under load, your UART
will not function properly?

Surely you don't mean that.