netdev - Re: [RFC PATCH 00/12] Implement XDP bpf

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Date:   Mon, 17 Jul 2017 10:04:44 -0700
From:   Jesse Brandeburg <jesse.brandeburg@...el.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     John Fastabend <john.fastabend@...il.com>,
        David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>,
        <andy@...yhouse.net>, <daniel@...earbox.net>, <ast@...com>,
        <alexander.duyck@...il.com>, <bjorn.topel@...el.com>,
        <jakub.kicinski@...ronome.com>, <ecree@...arflare.com>,
        <sgoutham@...ium.com>, <Yuval.Mintz@...ium.com>,
        <saeedm@...lanox.com>, "Andi Kleen" <andi@...stfloor.org>
Subject: Re: [RFC PATCH 00/12] Implement XDP bpf_redirect vairants

On Sun, 16 Jul 2017 10:23:08 +0200
Jesper Dangaard Brouer <brouer@...hat.com> wrote:

> On Tue, 11 Jul 2017 12:37:10 -0700 John Fastabend <john.fastabend@...il.com> wrote:
> 
>  [...]  
> > 
> > hmm maybe Jesse or Alex have some clues. Adding them to the CC list.  
> 
> This seems related to Hyper-Threading.  I tried to disable
> hyperthreading in the BIOS, and the problem goes away.  That is, the
> benchmarks are no-longer affected by the CPU tuned-adm profile.

Wow, nice job figuring that out.  I went and took a look at tuned-adm
documentation but didn't see anything that stood out that would cause
the behavior you were seeing.

I suspect your toplev results showing this is a frontend problem mesh
nicely with the hyper-threading configuration inducing the bad behavior,
since there is still only one execution unit, and the fetchers would
have to stall.  I think you've rediscovered why the forwarding /
routing crowd generally turns off hyperthreading.