netdev - Re: ixgbe: ksoftirqd consumes 100% CPU w/ ~50 TCP conns

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Date:	Tue, 24 May 2016 12:46:56 -0700
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Brandon Philips <brandon@...p.co>
Cc:	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	John Fastabend <john.fastabend@...il.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Mark Rustad <mark.d.rustad@...el.com>,
	Netdev <netdev@...r.kernel.org>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: ixgbe: ksoftirqd consumes 100% CPU w/ ~50 TCP conns

I'm guessing the issue is lock contention on the IOMMU resource table.
I resolved most of that for the Rx side back when we implemented the
Rx page reuse but the Tx side still has to perform a DMA mapping for
each individual buffer.  Depending on the needs of the user if they
still need the IOMMU enabled for use with something like KVM one thing
they may try doing is use the kernel parameter "iommu=pt" to allow
host devices to access memory without the penalty for having to
allocate/free resources and still provide guests with IOMMU isolation.

- Alex

On Tue, May 24, 2016 at 9:40 AM, Brandon Philips <brandon@...p.co> wrote:
> Hello Everyone-
>
> So we tracked it down to IOMMU causing CPU affinity getting broken[1].
> Can we provide any further details or is this a known issue?
>
> Thank You,
>
> Brandon
>
> [1] https://github.com/coreos/bugs/issues/1275#issuecomment-219866601
>
> On Tue, May 17, 2016 at 12:44 PM, Brandon Philips <brandon@...p.co> wrote:
>> Hello ixgbe team-
>>
>> With Linux v4.6 and the ixgbe driver (details below) a user is reporting
>> ksoftirqd consuming 100% of the CPU on all cores after a moderate  ~20-50
>> number of TCP connections. They are unable to reproduce this issue with
>> Cisco hardware.
>>
>> With Kernel v3.19 they cannot reproduce[1] the issue. Disabling IOMMU
>> (intel_iommu=off) does "fix" the issue[2].
>>
>> Thank You,
>>
>> Brandon
>>
>> [1] https://github.com/coreos/bugs/issues/1275#issuecomment-219157803
>> [2] https://github.com/coreos/bugs/issues/1275#issuecomment-219819986
>>
>> Intel Corporation 82599ES 10-Gigabit SFI/SFP+ Network Connection (rev 01)
>> ethtool -i eno1
>> driver: ixgbe
>> version: 4.0.1-k
>> firmware-version: 0x800004e0
>> bus-info: 0000:06:00.0
>> supports-statistics: yes
>> supports-test: yes
>> supports-eeprom-access: yes
>> supports-register-dump: yes
>> supports-priv-flags: no
>>
>> CPU
>> Intel(R) Xeon(R) CPU E5-2690 v3 @ 2.60GHz