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

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Message-ID: <20201023085826.GP2611@hirez.programming.kicks-ass.net>
Date:   Fri, 23 Oct 2020 10:58:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nitesh Narayan Lal <nitesh@...hat.com>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>, helgaas@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        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,
        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 Thu, Oct 22, 2020 at 01:47:14PM -0400, Nitesh Narayan Lal wrote:

> Hi Peter,
> 
> So based on the suggestions from you and Thomas, I think something like the
> following should do the job within pci_alloc_irq_vectors_affinity():
> 
> +       if (!pci_is_managed(dev) && (hk_cpus < num_online_cpus()))
> +               max_vecs = clamp(hk_cpus, min_vecs, max_vecs);
> 
> I do know that you didn't like the usage of "hk_cpus < num_online_cpus()"
> and to an extent I agree that it does degrade the code clarity.

It's not just code clarity; I simply don't understand it. It feels like
a band-aid that breaks thing.

At the very least it needs a ginormous (and coherent) comment that
explains:

 - the interface
 - the usage
 - this hack

> However, since there is a certain inconsistency in the number of vectors
> that drivers request through this API IMHO we will need this, otherwise
> we could cause an impact on the drivers even in setups that doesn't
> have any isolated CPUs.

So shouldn't we then fix the drivers / interface first, to get rid of
this inconsistency?

> If you agree, I can send the next version of the patch-set.

Well, it's not just me you have to convince.