From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH] net-tun: restructure tun_do_read for better sleep/wakeup efficiency Date: Tue, 13 May 2014 11:20:27 +0300 Message-ID: <20140513082027.GB29442@redhat.com> References: <1399422244-22751-1-git-send-email-xii@google.com> <5369AB36.6030609@redhat.com> <536C4733.9020704@redhat.com> <20140512061557.GA12581@redhat.com> <5371B87D.3020609@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Xi Wang , "David S. Miller" , netdev@vger.kernel.org, Maxim Krasnyansky , Neal Cardwell , Eric Dumazet To: Jason Wang Return-path: Received: from mx1.redhat.com ([209.132.183.28]:16515 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752692AbaEMIVk (ORCPT ); Tue, 13 May 2014 04:21:40 -0400 Content-Disposition: inline In-Reply-To: <5371B87D.3020609@redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: On Tue, May 13, 2014 at 02:15:25PM +0800, Jason Wang wrote: > On 05/12/2014 02:15 PM, Michael S. Tsirkin wrote: > > On Fri, May 09, 2014 at 11:10:43AM +0800, Jason Wang wrote: > >> > On 05/09/2014 02:22 AM, Xi Wang wrote: > >>> > > On Tue, May 6, 2014 at 8:40 PM, Jason Wang wrote: > >>>> > >> On 05/07/2014 08:24 AM, Xi Wang wrote: > >>>>> > >>> tun_do_read always adds current thread to wait queue, even if a packet > >>>>> > >>> is ready to read. This is inefficient because both sleeper and waker > >>>>> > >>> want to acquire the wait queue spin lock when packet rate is high. > >>>> > >> After commit 61a5ff15ebdab87887861a6b128b108404e4706d, this will only > >>>> > >> help for blocking read. Looks like for performance critical userspaces, > >>>> > >> they will use non blocking reads. > >>>>> > >>> We restructure the read function and use common kernel networking > >>>>> > >>> routines to handle receive, sleep and wakeup. With the change > >>>>> > >>> available packets are checked first before the reading thread is added > >>>>> > >>> to the wait queue. > >>>> > >> This is interesting, since it may help if we want to add rx busy loop > >>>> > >> for tun. (In fact I worked a similar patch like this). > >>> > > > >>> > > Yes this should be a good side effect and I am also interested in trying. > >>> > > Busy polling in user space is not ideal as it doesn't give the lowest latency. > >>> > > Besides differences in interrupt latency etc., there is a bad case for > >>> > > non blocking mode: When a packet arrives right before the polling thread > >>> > > returns to userspace. The control flow has to cross kernel/userspace > >>> > > boundary 3 times before the packet can be processed, while kernel > >>> > > blocking or busy polling only needs 1 boundary crossing. > >> > > >> > So if we want to implement this, we need a feature bit to turn it on. > >> > Then vhost may benefit from this. > > IFF_TUN_POLL_BUSY_LOOP ? I'm not sure it has to be > > a flag. Maybe an ioctl is better, if userspace > > misconfigures this it is only hurting itself, right? > > Flag has the same effect. But adding new ioctls means userspace needs to > be modified. This is different with current rx busy polling for tcp/udp > socket which is transparent to userspace application. OTOH risk is much lower though. > > Maybe add a module parameter to control polling timeout, > > or reuse low_latency_poll. > > > > If we don't need a global parameter, we can just implement it without > generic helper like __skb_recv_datagram(). not sure I get the meaning here.