From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [PATCH net-next 2/2] net: exit busy loop when another process is runnable Date: Tue, 02 Sep 2014 11:38:40 +0800 Message-ID: <54053BC0.6060700@redhat.com> References: <1408608310-13579-1-git-send-email-jasowang@redhat.com> <1408608310-13579-2-git-send-email-jasowang@redhat.com> <1408683665.5648.69.camel@marge.simpson.net> <20140901093159.GB27892@worktop.ger.corp.intel.com> <20140901095219.GD21269@redhat.com> <20140901100434.GD27892@worktop.ger.corp.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: Mike Galbraith , davem@davemloft.net, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Ingo Molnar To: Peter Zijlstra , "Michael S. Tsirkin" Return-path: In-Reply-To: <20140901100434.GD27892@worktop.ger.corp.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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?