netdev - Re: mlx5 broken affinity

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Date:   Wed, 8 Nov 2017 11:19:28 -0500
From:   Jes Sorensen <jsorensen@...com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Sagi Grimberg <sagi@...mberg.me>
CC:     Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@....mellanox.co.il>,
        Networking <netdev@...r.kernel.org>,
        Leon Romanovsky <leonro@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Kernel Team <kernel-team@...com>,
        Christoph Hellwig <hch@....de>
Subject: Re: mlx5 broken affinity

On 11/07/2017 10:07 AM, Thomas Gleixner wrote:
> On Sun, 5 Nov 2017, Sagi Grimberg wrote:
>> I do agree that the user would lose better cpu online/offline behavior,
>> but it seems that users want to still have some control over the IRQ
>> affinity assignments even if they lose this functionality.
> 
> Depending on the machine and the number of queues this might even result in
> completely losing the ability to suspend/hibernate because the number of
> available vectors on CPU0 is not sufficient to accomodate all queue
> interrupts.

Depending on the system, suspend/resume is really the lesser interesting
issue to the user. Pretty much any system with a 10/25GBps mlx5 NIC in
it will be in some sort of rack and is unlikely to ever want to
suspend/resume.

>> Would it be possible to keep the managed facility until a user overrides
>> an affinity assignment? This way if the user didn't touch it, we keep
>> all the perks, and in case the user overrides it, we log the implication
>> so the user is aware?
> 
> A lot of things are possible, the question is whether it makes sense. The
> whole point is to have resources (queues, interrupts etc.) per CPU and have
> them strictly associated.
> 
> Why would you give the user a knob to destroy what you carefully optimized?
> 
> Just because we can and just because users love those knobs or is there any
> real technical reason?

Because the user sometimes knows better based on statically assigned
loads, or the user wants consistency across kernels. It's great that the
system is better at allocating this now, but we also need to allow for a
user to change it. Like anything on Linux, a user wanting to blow off
his/her own foot, should be allowed to do so.

Cheers,
Jes