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

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Date:	Tue, 2 Sep 2014 11:31:24 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>
Cc:	Jason Wang <jasowang@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	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 Tue, Sep 02, 2014 at 09:15:18AM +0300, 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. This is not something that plays nice with
> other apps, but this is what the user wants.

I think most applications wouldn't interpret this flag as "burn up CPU I don't
care what is the result", what apps want is more of "maximise throughput
and minimise latency even if throughput/CPU ratio goes down".
Jason posted benchmarks that show throughput going up because other
processes get more of a chance to run, so this seems consistent
with that goal.


> 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.
> 
> 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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