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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 1 Sep 2014 12:52:19 +0300

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?

> All in all, a complete trainwreck.
> 
> NAK.