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Date: Tue, 4 Feb 2014 15:35:02 +0100 From: Mathias Kretschmer <mathias.kretschmer@...us.fraunhofer.de> To: Daniel Borkmann <dborkman@...hat.com> CC: <netdev@...r.kernel.org>, "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>, Felix Fietkau <nbd@...nwrt.org>, Jesper Dangaard Brouer <jbrouer@...hat.com> Subject: Re: linux-3.14-rc1 & PACKET_QDISC_BYPASS : slow_path warning 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 -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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