Re: [PATCH net 1/3] igb: Disable threaded IRQ for igb_msix_other

[Simon Horman <horms@kernel.org>]

On Mon, Oct 21, 2024 at 04:26:24PM -0700, Jacob Keller wrote:
> From: Wander Lairson Costa <wander@redhat.com>
> 
> During testing of SR-IOV, Red Hat QE encountered an issue where the
> ip link up command intermittently fails for the igbvf interfaces when
> using the PREEMPT_RT variant. Investigation revealed that
> e1000_write_posted_mbx returns an error due to the lack of an ACK
> from e1000_poll_for_ack.
> 
> The underlying issue arises from the fact that IRQs are threaded by
> default under PREEMPT_RT. While the exact hardware details are not
> available, it appears that the IRQ handled by igb_msix_other must
> be processed before e1000_poll_for_ack times out. However,
> e1000_write_posted_mbx is called with preemption disabled, leading
> to a scenario where the IRQ is serviced only after the failure of
> e1000_write_posted_mbx.
> 
> To resolve this, we set IRQF_NO_THREAD for the affected interrupt,
> ensuring that the kernel handles it immediately, thereby preventing
> the aforementioned error.
> 
> Reproducer:
> 
>     #!/bin/bash
> 
>     # echo 2 > /sys/class/net/ens14f0/device/sriov_numvfs
>     ipaddr_vlan=3
>     nic_test=ens14f0
>     vf=${nic_test}v0
> 
>     while true; do
> 	    ip link set ${nic_test} mtu 1500
> 	    ip link set ${vf} mtu 1500
> 	    ip link set $vf up
> 	    ip link set ${nic_test} vf 0 vlan ${ipaddr_vlan}
> 	    ip addr add 172.30.${ipaddr_vlan}.1/24 dev ${vf}
> 	    ip addr add 2021:db8:${ipaddr_vlan}::1/64 dev ${vf}
> 	    if ! ip link show $vf | grep 'state UP'; then
> 		    echo 'Error found'
> 		    break
> 	    fi
> 	    ip link set $vf down
>     done
> 
> Signed-off-by: Wander Lairson Costa <wander@redhat.com>
> Fixes: 9d5c824399de ("igb: PCI-Express 82575 Gigabit Ethernet driver")
> Reported-by: Yuying Ma <yuma@redhat.com>
> Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
> Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
> Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>

Reviewed-by: Simon Horman <horms@kernel.org>