On Mon, Sep 01, 2014 at 12:04:34PM +0200, Peter Zijlstra wrote: > 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,
Yea, it's a kludge, but busy waiting is a heuristic thing anyway, so it boils down to whether it's mostly effective. I agree it would be better to make it more robust/consistent if we can do it without a lot of complexity. > but let me reread this patch > description to see if it explains why that is a good thing. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
