From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752866AbaIAJcE (ORCPT ); Mon, 1 Sep 2014 05:32:04 -0400 Received: from casper.infradead.org ([85.118.1.10]:56056 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752479AbaIAJcC (ORCPT ); Mon, 1 Sep 2014 05:32:02 -0400 Date: Mon, 1 Sep 2014 11:31:59 +0200 From: Peter Zijlstra To: Mike Galbraith Cc: Jason Wang , davem@davemloft.net, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, mst@redhat.com, Ingo Molnar Subject: Re: [PATCH net-next 2/2] net: exit busy loop when another process is runnable Message-ID: <20140901093159.GB27892@worktop.ger.corp.intel.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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1408683665.5648.69.camel@marge.simpson.net> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. All in all, a complete trainwreck. NAK.