On 09/01/2014 06:04 PM, Peter Zijlstra wrote:

On Mon, Sep 01, 2014 at 12:52:19PM +0300, Michael S. Tsirkin wrote:

quoted

On Mon, Sep 01, 2014 at 11:31:59AM +0200, Peter Zijlstra wrote:

quoted

On Fri, Aug 22, 2014 at 07:01:05AM +0200, Mike Galbraith wrote:

quoted

quoted

+++ 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.

I see, how about just exporting a boolean helper like
current_can_busy_loop() and take care all of the conditions (pending bhs
and rcu callbacks, runnable processes) in scheduler code itself?