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

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Date:	Tue, 02 Sep 2014 15:37:23 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>
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

On 09/02/2014 02:15 PM, Eliezer Tamir wrote:
> On 02/09/2014 06:29, Jason Wang wrote:
>> On 09/01/2014 02:39 PM, Eliezer Tamir wrote:
>>> On 29/08/2014 06:08, Jason Wang wrote:
>>>>> Yes, but rx busy polling only works in process context and does not
>>>>> disable bh, so it may be not an issue.
>>> sk_busy_loop() uses rcu_read_lock_bh(), so it does run with bh disabled.
>> True, so we need probably also exit the loop when there are pending bhs.
> I'm not so sure, in the typical busy poll scenario, the incoming
> traffic is the most time-critical thing in the system.
> It's so important that you are willing to trade lots of CPU power
> for better latency. The user has decided that he wants to dedicate
> this CPU mostly for that. 

But user should increase the process priority or cgroup in this case.
> This is not something that plays nice with
> other apps, but this is what the user wants.

So the busy polling looks have a higher priority somehow than other
processes.
> So, you definitely don't want to starve any bh, and you should
> regularly re-enable bh's, but you also don't want to stop everything
> at any time a bh is scheduled.

If I get your meaning, you may want call to rcu_read_lock_bh() and get
socket napi id inside the do{} loop? This seems can prevent bhs from
being starved and can also handle the case that the packets were from
different NAPIs.
>
> You also want network processing on the queues that are busy polled
> to come through busy polling and not through NAPI, which is run in bh
> context.
>
> -Eliezer
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