From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753204AbaIAJxC (ORCPT ); Mon, 1 Sep 2014 05:53:02 -0400 Received: from mx1.redhat.com ([209.132.183.28]:30214 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752991AbaIAJxA (ORCPT ); Mon, 1 Sep 2014 05:53:00 -0400 Date: Mon, 1 Sep 2014 12:52:19 +0300 From: "Michael S. Tsirkin" To: Peter Zijlstra Cc: Mike Galbraith , Jason Wang , davem@davemloft.net, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Ingo Molnar Subject: Re: [PATCH net-next 2/2] net: exit busy loop when another process is runnable Message-ID: <20140901095219.GD21269@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140901093159.GB27892@worktop.ger.corp.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? > All in all, a complete trainwreck. > > NAK.