On 09/01/2014 06:04 PM, 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, 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? -- 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/
