Re: [RFC 0/3] softirq: Introduce softirq throttling

[Matthew Wilcox]

On Wed, Apr 06, 2022 at 10:52:38AM +0800, Liao Chang wrote:
> Kernel check for pending softirqs periodically, they are performed in a
> few points of kernel code, such as irq_exit() and __local_bh_enable_ip(),
> softirqs that have been activated by a given CPU must be executed on the
> same CPU, this characteristic of softirq is always a potentially
> "dangerous" operation, because one CPU might be end up very busy while
> the other are most idle.
> 
> Above concern is proven in a networking user case: recenlty, we
> engineer find out the time used for connection re-establishment on
> kernel v5.10 is 300 times larger than v4.19, meanwhile, softirq
> monopolize almost 99% of CPU. This problem stem from that the connection
> between Sender and Receiver node get lost, the NIC driver on Sender node
> will keep raising NET_TX softirq before connection recovery. The system
> log show that most of softirq is performed from __local_bh_enable_ip(),
> since __local_bh_enable_ip is used widley in kernel code, it is very
> easy to run out most of CPU, and the user-mode application can't obtain
> enough CPU cycles to establish connection as soon as possible.

Shouldn't you fix that bug instead?  This seems like papering over the
bad effects of a bug and would make it harder to find bugs like this in
the future.  Essentially, it's the same as a screaming hardware interrupt,
except that it's a software interrupt, so we can fix the bug instead of
working around broken hardware.