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

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Date:   Thu, 17 Sep 2020 11:23:59 -0700
From:   Jesse Brandeburg <jesse.brandeburg@...el.com>
To:     Nitesh Narayan Lal <nitesh@...hat.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>
Subject: Re: [RFC][Patch v1 2/3] i40e: limit msix vectors based on
 housekeeping CPUs

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?  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:

$ 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>