netdev - Re: mlx5 broken affinity

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Date:   Thu, 2 Nov 2017 10:48:07 -0400
From:   Jes Sorensen <jsorensen@...com>
To:     Sagi Grimberg <sagi@...mberg.me>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@....mellanox.co.il>
CC:     Networking <netdev@...r.kernel.org>,
        Leon Romanovsky <leonro@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Kernel Team <kernel-team@...com>,
        Christoph Hellwig <hch@....de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: mlx5 broken affinity

On 11/02/2017 06:08 AM, Sagi Grimberg wrote:
> 
>>>> I vaguely remember Nacking Sagi's patch as we knew it would break
>>>> mlx5e netdev affinity assumptions.
>> I remember that argument. Still the series found its way in.
> 
> Of course it maid its way in, it was acked by three different
> maintainers, and I addressed all of Saeed's comments.
> 
>> That series moves affinity decisions to kernel's responsibility.
>> AFAI see, what kernel does is assign IRQs to the NUMA's one by one in
>> increasing indexing (starting with cores of NUMA #0), no matter what
>> NUMA is closer to the NIC.
> 
> Well, as we said before, if there is a good argument to do the home node
> first we can change the generic code (as it should be given that this is
> absolutely not device specific).
> 
>> This means that if your NIC is on NUMA #1, and you reduce the number
>> of channels, you might end up working only with the cores on the far
>> NUMA. Not good!
> We deliberated on this before, and concluded that application affinity
> and device affinity are equally important. If you have a real use case
> that shows otherwise, its perfectly doable to start from the device home
> node.

This wasn't to start a debate about which allocation method is the
perfect solution. I am perfectly happy with the new default, the part
that is broken is to take away the user's option to reassign the
affinity. That is a bug and it needs to be fixed!

Jes