Return-path: Received: from mx-relay06-dus.antispameurope.com ([94.100.134.206]:48795 "EHLO mx-relay06-dus.antispameurope.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752794AbaBDOfU (ORCPT ); Tue, 4 Feb 2014 09:35:20 -0500 Message-ID: <52F0FA96.7090903@fokus.fraunhofer.de> (sfid-20140204_153529_116541_F702E88B) Date: Tue, 4 Feb 2014 15:35:02 +0100 From: Mathias Kretschmer MIME-Version: 1.0 To: Daniel Borkmann CC: , "linux-wireless@vger.kernel.org" , Felix Fietkau , Jesper Dangaard Brouer Subject: Re: linux-3.14-rc1 & PACKET_QDISC_BYPASS : slow_path warning References: <52F01C97.20001@fokus.fraunhofer.de> <52F0E361.9000304@redhat.com> <52F0E771.8070904@fokus.fraunhofer.de> <52F0F86C.3050402@redhat.com> In-Reply-To: <52F0F86C.3050402@redhat.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Sender: linux-wireless-owner@vger.kernel.org List-ID: On 02/04/2014 03:25 PM, Daniel Borkmann wrote: > On 02/04/2014 02:13 PM, Mathias Kretschmer wrote: >> On 02/04/2014 01:56 PM, Daniel Borkmann wrote: >>> On 02/03/2014 11:47 PM, Mathias Kretschmer wrote: > ... >>>> we are developing a wired/wireless MPLS switch. Currently the data plane runs in >>>> user space using PF_PACKET sockets via RX_RING/TX_RING. >>>> >>>> We had hoped to test the PACKET_QDISC_BYPASS option since this seems to be the >>>> proper optimization for our purposes. >>>> >>>> Unfortunately, we're seeing a 'slow path' warning for every packet that is being >>>> sent out. With PACKET_QDISC_BYPASS disabled, no warnings are dumped. Hardware is >>>> an older AMD Geode LX embedded board (ALiX). >>>> >>>> BTW, this happens while sending via a wireless (802.11) adhoc interface. Hence, it >>>> might be an interaction with the ieee80211 sub system. >>> >>> Hm, so the WARN_ON() is triggered inside ath9k driver in relation to 802.11 QoS, >>> and came in from commit 066dae93bdf ("ath9k: rework tx queue selection and fix >>> queue stopping/waking"). We did the stress testing of that option for PF_PACKET >>> on 10Gbit/s NICs. Seems to me you might be running into the same issue when using >>> pktgen as it randomly or per round-robin selects tx queues as well? Not entirely >>> sure how necessary this WARN_ON() is though, Felix? I think QDISC_BYPASS might not >>> be the best option in your case, perhaps you will run into increased power usage >>> in your NIC as a side-effect? >> >> I'm not familiar with the exact implementation details, but from the description >> of this option, it seems to me that this is exactly what one would want to use if >> the goal is to send an Ethernet frame out on a particular interface without any >> further processing by the kernel. >> >> Why would this increase the power usage on the NIC ? Due to a higher achievable >> packet rate ? That would be acceptable :) > > I'm not too familiar with the ieee80211 sub system, so I let Felix answer side > effects and if actually the WARN_ON() is needed. ;) PACKET_QDISC_BYPASS is, as > documented, designed for advanced pktgen resp. traffic generator like scenarios > where you just sort of "brute force" packets to your NIC to stress test a remote > machine for further analysis. I don't think it's very useful in your scenario > when you have a wired/wireless MPLS switch, you rather might want to buffer/queue > and therefore use qdisc layer instead. Hm, I was hoping/assuming that we still get to use hardware queues, if provided by the driver. The main goal was to avoid any further PF_PACKET framework overhead. If the WARN_ON() issue gets solved, we will revisit this option and evaluate its applicability. Thanks, Mathias