netdev - Re: mlx5 broken affinity

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Message-ID: <6ee51a50-81a6-61d6-2e08-f9d4ffb3aacf@fb.com>
Date:   Thu, 9 Nov 2017 08:19:37 -0700
From:   Jens Axboe <axboe@...com>
To:     Sagi Grimberg <sagi@...mberg.me>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jes Sorensen <jsorensen@...com>
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 11/09/2017 03:50 AM, Sagi Grimberg wrote:
> Thomas,
> 
>>> 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.
>>
>> That's fine, but that's not what the managed affinity facility provides. If
>> you want to leverage the spread mechanism, but avoid the managed part, then
>> this is a different story and we need to figure out how to provide that
>> without breaking the managed side of it.
>>
>> As I said it's possible, but I vehemently disagree, that this is a bug in
>> the core code, as it was claimed several times in this thread.
>>
>> The real issue is that the driver was converted to something which was
>> expected to behave differently. That's hardly a bug in the core code, at
>> most it's a documentation problem.
> 
> I disagree here, this is not a device specific discussion. The question
> of exposing IRQ affinity assignment knob to user-space holds for every
> device (NICs, HBAs and alike). The same issue could have been raised on
> any other device using managed affinitization (and we have quite a few
> of those now). I can't see any reason why its OK for device X to use it
> while its not OK for device Y to use it.
> 
> If the resolution is "YES we must expose a knob to user-space" then the
> managed facility is unusable in its current form for *any* device. If
> the answer is "NO, user-space can't handle all the stuff the kernel can"
> then we should document it. This is really device independent.

Completely agree, and honestly I'm pretty baffled we're even having
to argue this point.

-- 
Jens Axboe