linux-kernel - Re: [2.6.36.1] IGB driver handles all ethX interrupts on single cpu core.

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Date:	Wed, 08 Dec 2010 08:59:25 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Paweł Sikora <pluto@...k.net>
Cc:	Robert Hancock <hancockrwd@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [2.6.36.1] IGB driver handles all ethX interrupts on single
 cpu core.

Le mercredi 08 décembre 2010 à 08:45 +0100, Paweł Sikora a écrit :
> On Wednesday 08 of December 2010 02:44:25 Robert Hancock wrote:
> > On 12/07/2010 12:06 PM, Paweł Sikora wrote:
> > > hi,
> > >
> > > i'm currently testing a new server with 2x opteron-6128 with dual gigabit port
> > > and observing that the igb driver uses only single core for all ethX interrupts.
> > > is it a correct behaviour for this driver?
> > >
> > > BR,
> > > Pawel.
> > 
> > The CPU affinity for the IRQ isn't really under the driver's control. It 
> > looks like all your interrupts are being handled on CPU0. You likely 
> > need to run the irqbalance daemon.
> 
> ok, so why e.g. on one machine (dual amd opteron) irqbalance daemon is required
> and on second machine (single intel quad-core) irqs are balanced w/o daemon?
> this looks inconsistent to me.
> 
> $ cat /proc/interrupts
>            CPU0       CPU1       CPU2       CPU3
>   0:         49          2          0          0   IO-APIC-edge      timer
>   1:          0          1          0          1   IO-APIC-edge      i8042
>   8:         13         11         12         13   IO-APIC-edge      rtc0
>   9:          0          0          0          0   IO-APIC-fasteoi   acpi
>  12:          0          0          3          1   IO-APIC-edge      i8042
>  16:        224        236        235        232   IO-APIC-fasteoi   pata_marvell, uhci_hcd:usb3
>  17:          0          0          0          0   IO-APIC-fasteoi   saa7133[0], saa7133[0]
>  18:          0          0          0          0   IO-APIC-fasteoi   ehci_hcd:usb1, uhci_hcd:usb5, uhci_hcd:usb8
>  19:        695        672        660        630   IO-APIC-fasteoi   uhci_hcd:usb7
>  21:          0          0          0          0   IO-APIC-fasteoi   uhci_hcd:usb4
>  23:          1          1          1          0   IO-APIC-fasteoi   ehci_hcd:usb2, uhci_hcd:usb6
>  40:       3409       3446       3441       3403   PCI-MSI-edge      ahci
>  41:         63         60         60         61   PCI-MSI-edge      hda_intel
>  42:       3219       3180       3237       3192   PCI-MSI-edge      radeon
>  43:        505        487        496        498   PCI-MSI-edge      eth0


irqbalance is not required.

You can decide yourself of IRQ affinities

Given data you provided :

echo 0f >/proc/irq/*/eth0/../smp_affinity

If your IGB card has 4 queues and you want each queue serviced by a
given cpu :

echo 01 >/proc/irq/*/eth0-fp-0/../smp_affinity
echo 02 >/proc/irq/*/eth0-fp-1/../smp_affinity
echo 04 >/proc/irq/*/eth0-fp-2/../smp_affinity
echo 08 >/proc/irq/*/eth0-fp-3/../smp_affinity

Note 1 : If under load, NAPI is triggered and no anymore hardirqs
are delivered, so load stay on one ksoftirqd (one cpu),
no matter changes you make on /proc/irq/...



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