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

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Message-ID: <20140901093159.GB27892@worktop.ger.corp.intel.com>
Date:	Mon, 1 Sep 2014 11:31:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Jason Wang <jasowang@...hat.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	mst@...hat.com, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH net-next 2/2] net: exit busy loop when another process is
 runnable

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.

All in all, a complete trainwreck.

NAK.
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