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

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Date:	Mon, 1 Sep 2014 12:04:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Jason Wang <jasowang@...hat.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH net-next 2/2] net: exit busy loop when another process is
 runnable

On Mon, Sep 01, 2014 at 12:52:19PM +0300, Michael S. Tsirkin wrote:
> On Mon, Sep 01, 2014 at 11:31:59AM +0200, Peter Zijlstra wrote:
> > On Fri, Aug 22, 2014 at 07:01:05AM +0200, Mike Galbraith wrote:
> > > > +++ b/include/net/busy_poll.h
> > > > @@ -109,7 +109,8 @@ static inline bool sk_busy_loop(struct sock *sk, int nonblock)
> > > >  		cpu_relax();
> > > >  
> > > >  	} while (!nonblock && skb_queue_empty(&sk->sk_receive_queue) &&
> > > > -		 !need_resched() && !busy_loop_timeout(end_time));
> > > > +		 !need_resched() && !busy_loop_timeout(end_time) &&
> > > > +		 nr_running_this_cpu() < 2);
> > > >  
> > 
> > So as has been said by now; this is horrible.
> > 
> > We should not export nr_running like this ever. Your usage of < 2
> > implies this can be hit with nr_running == 0, and therefore you can also
> > hit it with nr_running == 1 where the one is not network related and you
> > get random delays.
> > 
> > Worse still, you have BH (and thereby preemption) disabled, you should
> > not _ever_ have undefined and indefinite waits like that.
> > 
> > You also destroy any hope of dropping into lower power states; even when
> > there's never going to be a packet ever again, also bad.
> 
> Hmm this patch sometimes makes us exit from the busy loop *earlier*.
> How can this interfere with dropping into lower power states?

Ah.. jetlag.. :/ I read it like it owuld indefinitely spin if there was
only the 'one' task, not avoid the spin unless there was the one task.

The nr_running thing is still horrible, but let me reread this patch
description to see if it explains why that is a good thing.
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