linux-kernel - Re: [PATCH v3 4/4] PCI: Limit pci_alloc_irq

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Date:   Fri, 25 Sep 2020 19:18:11 -0400
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        linux-pci@...r.kernel.org, intel-wired-lan@...ts.osuosl.org,
        frederic@...nel.org, mtosatti@...hat.com, sassmann@...hat.com,
        jesse.brandeburg@...el.com, lihong.yang@...el.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, jiri@...dia.com, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, lgoncalv@...hat.com
Subject: Re: [PATCH v3 4/4] PCI: Limit pci_alloc_irq_vectors() to housekeeping
 CPUs


On 9/25/20 5:38 PM, Nitesh Narayan Lal wrote:
> On 9/25/20 4:23 PM, Bjorn Helgaas wrote:

[...]

>>> +	/*
>>> +	 * If we have isolated CPUs for use by real-time tasks, to keep the
>>> +	 * latency overhead to a minimum, device-specific IRQ vectors are moved
>>> +	 * to the housekeeping CPUs from the userspace by changing their
>>> +	 * affinity mask. Limit the vector usage to keep housekeeping CPUs from
>>> +	 * running out of IRQ vectors.
>>> +	 */
>>> +	if (hk_cpus < num_online_cpus()) {
>>> +		if (hk_cpus < min_vecs)
>>> +			max_vecs = min_vecs;
>>> +		else if (hk_cpus < max_vecs)
>>> +			max_vecs = hk_cpus;
>>> +	}
>> It seems like you'd want to do this inside
>> pci_alloc_irq_vectors_affinity() since that's an exported interface,
>> and drivers that use it will bypass the limiting you're doing here.
> Good point, few drivers directly use this.
> I took a quick look and it seems I may also have to take the pre and the
> post vectors into consideration.
>

It seems my initial interpretation was incorrect, reserved vecs (pre+post)
should always be less than the min_vecs.
So, FWIU we should be fine in limiting the max_vecs.

I can make this change and do a repost.

Do you have any other concerns with this patch or with any of the other
patches?

[...]

-- 
Nitesh



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