linux-kernel - Re: [PATCH] net/mlx5: Use cpumask_local

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Message-ID: <1a3208e3-a692-4d72-8705-7367b0bc4468@gmail.com>
Date: Thu, 15 Aug 2024 13:39:51 +0300
From: Tariq Toukan <ttoukan.linux@...il.com>
To: Yury Norov <yury.norov@...il.com>, Jakub Kicinski <kuba@...nel.org>,
 Erwan Velu <erwanaliasr1@...il.com>
Cc: Erwan Velu <e.velu@...teo.com>, Saeed Mahameed <saeedm@...dia.com>,
 Leon Romanovsky <leon@...nel.org>, Tariq Toukan <tariqt@...dia.com>,
 Yury Norov <ynorov@...dia.com>, Rahul Anand <raanand@...dia.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
 linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net/mlx5: Use cpumask_local_spread() instead of custom
 code

On 14/08/2024 17:45, Yury Norov wrote:
> On Wed, Aug 14, 2024 at 10:48:40AM +0300, Tariq Toukan wrote:
>>
>>
>> On 12/08/2024 11:22, Erwan Velu wrote:
>>> Commit 2acda57736de ("net/mlx5e: Improve remote NUMA preferences used for the IRQ affinity hints")
>>> removed the usage of cpumask_local_spread().
>>>
>>> The issue explained in this commit was fixed by
>>> commit 406d394abfcd ("cpumask: improve on cpumask_local_spread() locality").
>>>
>>> Since this commit, mlx5_cpumask_default_spread() is having the same
>>> behavior as cpumask_local_spread().
>>>
>>
>> Adding Yuri.
>>
>> One patch led to the other, finally they were all submitted within the same
>> patchset.
>>
>> cpumask_local_spread() indeed improved, and AFAIU is functionally equivalent
>> to existing logic.
>> According to [1] the current code is faster.
>> However, this alone is not a relevant enough argument, as we're talking
>> about slowpath here.
>>
>> Yuri, is that accurate? Is this the only difference?
>>
>> If so, I am fine with this change, preferring simplicity.
>>
>> [1] https://elixir.bootlin.com/linux/v6.11-rc3/source/lib/cpumask.c#L122
> 
> If you end up calling mlx5_cpumask_default_spread() for each CPU, it
> would be O(N^2). If you call cpumask_local_spread() for each CPU, your
> complexity would be O(N*logN), because under the hood it uses binary
> search.
> 
> The comment you've mentioned says that you can traverse your CPUs in
> O(N) if you can manage to put all the logic inside the
> for_each_numa_hop_mask() iterator. It doesn't seem to be your case.
> 
> I agree with you. mlx5_cpumask_default_spread() should be switched to
> using library code.
> 
> Acked-by: Yury Norov <yury.norov@...il.com>
> 
> You may be interested in siblings-aware CPU distribution I've made
> for mana ethernet driver in 91bfe210e196. This is also an example
> where using for_each_numa_hop_mask() over simple cpumask_local_spread()
> is justified.
> 
> Thanks,
> Yury

Thanks Yuri.

For the patch:
Reviewed-by: Tariq Toukan <tariqt@...dia.com>

Targeting net-next.
The patch subject should've t stated this clearly.

Jakub,
Please note that this patch is also mistakenly marked 'Not Applicable' 
already...

Regards,
Tariq