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Re: Irq architecture for multi-core network driver.

To: Stephen Hemminger <>
Subject: Re: Irq architecture for multi-core network driver.
From: David Daney <>
Date: Wed, 16 Dec 2009 15:26:02 -0800
Cc: Chetan Loke <>, Chris Friesen <>,, Linux Kernel Mailing List <>, linux-mips <>
In-reply-to: <20091216150051.63b6e31c@nehalam>
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Stephen Hemminger wrote:
On Wed, 16 Dec 2009 14:30:36 -0800
David Daney <> wrote:

Chetan Loke wrote:
Does your hardware do flow-based queues?  In this model you have
multiple rx queues and the hardware hashes incoming packets to a single
queue based on the addresses, ports, etc. This ensures that all the
packets of a single connection always get processed in the order they
arrived at the net device.

Indeed, this is exactly what we have.

Typically in this model you have as many interrupts as queues
(presumably 16 in your case).  Each queue is assigned an interrupt and
that interrupt is affined to a single core.
Certainly this is one mode of operation that should be supported, but I
would also like to be able to go for raw throughput and have as many cores
as possible reading from a single queue (like I currently have).

Well, you could let the NIC firmware(f/w) handle this. The f/w would
know which interrupt was just injected recently.In other words it
would have a history of which CPU's would be available. So if some
previously interrupted CPU isn't making good progress then the
firmware should route the incoming response packets to a different
queue. This way some other CPU will pick it up.

It isn's a NIC. There is no firmware. The system interrupt hardware is what it is and cannot be changed.

My current implementation still has a single input queue configured and I get a maskable interrupt on a single CPU when packets are available. If the queue depth increases above a given threshold, I optionally send an IPI to another CPU to enable NAPI polling on that CPU.

Currently I have a module parameter that controls the maximum number of CPUs that will have NAPI polling enabled.

This allows me to get multiple CPUs doing receive processing without having to hack into the lower levels of the system's interrupt processing code to try to do interrupt steering. Since all the interrupt service routine was doing was call netif_rx_schedule(), I can simply do this via smp_call_function_single().

Better to look into receive packet steering patches that are still
under review (rather than reinventing it just for your driver)

Indeed. Although it turns out that I can do packet steering in hardware across up to 16 queues each with their own irq and thus dedicated CPU. So it is unclear to me if the receive packet steering patches offer much benefit to this hardware.

One concern is the ability to forward as many packets as possible from a very low number of flows (between 1 and 4). Since it is an artificial benchmark, we can arbitrarily say that packet reordering is allowed. The simple hack to do NAPI polling on all CPUs from a single queue gives good results. There is no need to remind me that packet reordering should be avoided, I already know this.

David Daney

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