linux-kernel - Re: [RFC][Patch v1 2/3] i40e: limit msix vectors based on housekeeping CPUs

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Date:   Thu, 17 Sep 2020 14:31:07 -0400
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     Jesse Brandeburg <jesse.brandeburg@...el.com>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        linux-pci@...r.kernel.org, frederic@...nel.org,
        mtosatti@...hat.com, sassmann@...hat.com,
        jeffrey.t.kirsher@...el.com, jacob.e.keller@...el.com,
        jlelli@...hat.com, hch@...radead.org, bhelgaas@...gle.com,
        mike.marciniszyn@...el.com, dennis.dalessandro@...el.com,
        thomas.lendacky@....com, jerinj@...vell.com,
        mathias.nyman@...el.com, jiri@...dia.com,
        "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Subject: Re: [RFC][Patch v1 2/3] i40e: limit msix vectors based on
 housekeeping CPUs


On 9/17/20 2:23 PM, Jesse Brandeburg wrote:
> Nitesh Narayan Lal wrote:
>
>> In a realtime environment, it is essential to isolate unwanted IRQs from
>> isolated CPUs to prevent latency overheads. Creating MSIX vectors only
>> based on the online CPUs could lead to a potential issue on an RT setup
>> that has several isolated CPUs but a very few housekeeping CPUs. This is
>> because in these kinds of setups an attempt to move the IRQs to the
>> limited housekeeping CPUs from isolated CPUs might fail due to the per
>> CPU vector limit. This could eventually result in latency spikes because
>> of the IRQ threads that we fail to move from isolated CPUs.
>>
>> This patch prevents i40e to add vectors only based on available
>> housekeeping CPUs by using num_housekeeping_cpus().
>>
>> Signed-off-by: Nitesh Narayan Lal <nitesh@...hat.com>
> The driver changes are straightforward, but this isn't the only driver
> with this issue, right?

Indeed, I was hoping to modify them over the time with some testing.

>   I'm sure ixgbe and ice both have this problem
> too, you should fix them as well, at a minimum, and probably other
> vendors drivers:

Sure, I can atleast include ixgbe and ice in the next posting if that makes
sense.
The reason I skipped them is that I was not very sure about the right way to
test these changes.

>
> $ rg -c --stats num_online_cpus drivers/net/ethernet
> ...
> 50 files contained matches
>
> for this patch i40e
> Acked-by: Jesse Brandeburg <jesse.brandeburg@...el.com>
>
-- 
Thanks
Nitesh



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