Subject: Re: [PATCH net-next 2/2] net: exit busy loop when another process is runnable

From: Ingo Molnar <[email-masked]>
Date: Fri, 22 Aug 2014 09:42:24 +0200

* Jason Wang <[email-masked]> wrote:

> Polling could be done by either rx busy loop in process 
> context or NAPI in softirq. [...]

Note that this shows another reason why it's a bad idea to 
query nr_running directly: depending on the softirq processing 
method, a softirq might run:

 - directly in process context
 - in an idle thread's context
 - or in a ksoftirqd context. 

'nr_running' will have different values in these cases, causing 
assymetries in busy-poll handling!

Another class of assymetry is when there are other softirq bits 
pending, beyond NET_RX (or NET_TX): a nr_running check misses 
them.

The solution I outlined in the previous mail (using a 
sched_expected_runtime() method) would be able to avoid most of 
these artifacts.

Thanks,

	Ingo