netdev - Re: [PATCH v4 4/4] PCI: Limit pci_alloc_irq

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Date:   Mon, 19 Oct 2020 10:25:14 -0400
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>,
        Peter Zijlstra <peterz@...radead.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, sassmann@...hat.com,
        jesse.brandeburg@...el.com, lihong.yang@...el.com,
        helgaas@...nel.org, 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, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, lgoncalv@...hat.com
Subject: Re: [PATCH v4 4/4] PCI: Limit pci_alloc_irq_vectors() to housekeeping
 CPUs


On 10/19/20 10:00 AM, Marcelo Tosatti wrote:
> On Mon, Oct 19, 2020 at 01:11:37PM +0200, Peter Zijlstra wrote:
>> On Sun, Oct 18, 2020 at 02:14:46PM -0400, Nitesh Narayan Lal wrote:

[...]

>>>> Also, do we really need to have that conditional on hk_cpus <
>>>> num_online_cpus()? That is, why can't we do this unconditionally?
>>> FWIU most of the drivers using this API already restricts the number of
>>> vectors based on the num_online_cpus, if we do it unconditionally we can
>>> unnecessary duplicate the restriction for cases where we don't have any
>>> isolated CPUs.
>> unnecessary isn't really a concern here, this is a slow path. What's
>> important is code clarity.

Right, I can skip that check then.

>>
>>> Also, different driver seems to take different factors into consideration
>>> along with num_online_cpus while finding the max_vecs to request, for
>>> example in the case of mlx5:
>>> MLX5_CAP_GEN(dev, num_ports) * num_online_cpus() +
>>>                MLX5_EQ_VEC_COMP_BASE
>>>
>>> Having hk_cpus < num_online_cpus() helps us ensure that we are only
>>> changing the behavior when we have isolated CPUs.
>>>
>>> Does that make sense?
>> That seems to want to allocate N interrupts per cpu (plus some random
>> static amount, which seems weird, but whatever). This patch breaks that.
> On purpose. For the isolated CPUs we don't want network device 
> interrupts (in this context).
>
>> So I think it is important to figure out what that driver really wants
>> in the nohz_full case. If it wants to retain N interrupts per CPU, and
>> only reduce the number of CPUs, the proposed interface is wrong.
> It wants N interrupts per non-isolated (AKA housekeeping) CPU.
> Zero interrupts for isolated interrupts.

Right, otherwise we may end up in a situation where we run out of per CPU
vectors while we move the IRQs from isolated CPUs to housekeeping.


-- 
Thanks
Nitesh



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