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Subject: Re: [RFC][Patch v1 2/3] i40e: limit msix vectors based on
 housekeeping CPUs
To:     Jesse Brandeburg <jesse.brandeburg@intel.com>
Cc:     linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        linux-pci@vger.kernel.org, frederic@kernel.org,
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        "Luis Claudio R. Goncalves" <lgoncalv@redhat.com>
References: <20200909150818.313699-1-nitesh@redhat.com>
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 <20200917112359.00006e10@intel.com>
From:   Nitesh Narayan Lal <nitesh@redhat.com>
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Message-ID: <9a73ab21-1ad6-9a08-b7e7-53ef85649563@redhat.com>
Date:   Thu, 17 Sep 2020 14:31:07 -0400
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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@redhat.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 t=
o
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@intel.com>
>
--=20
Thanks
Nitesh


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