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

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Message-ID: <20140822074224.GB7372@gmail.com>
Date:	Fri, 22 Aug 2014 09:42:24 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Jason Wang <jasowang@...hat.com>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	mst@...hat.com, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH net-next 2/2] net: exit busy loop when another process is
 runnable


* Jason Wang <jasowang@...hat.com> 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
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