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Date:   Fri, 23 Oct 2020 10:58:26 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Nitesh Narayan Lal <nitesh@redhat.com>
Cc:     Marcelo Tosatti <mtosatti@redhat.com>, helgaas@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        linux-pci@vger.kernel.org, intel-wired-lan@lists.osuosl.org,
        frederic@kernel.org, sassmann@redhat.com,
        jesse.brandeburg@intel.com, lihong.yang@intel.com,
        jeffrey.t.kirsher@intel.com, jacob.e.keller@intel.com,
        jlelli@redhat.com, hch@infradead.org, bhelgaas@google.com,
        mike.marciniszyn@intel.com, dennis.dalessandro@intel.com,
        thomas.lendacky@amd.com, jiri@nvidia.com, mingo@redhat.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        lgoncalv@redhat.com
Subject: Re: [PATCH v4 4/4] PCI: Limit pci_alloc_irq_vectors() to
 housekeeping CPUs
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On Thu, Oct 22, 2020 at 01:47:14PM -0400, Nitesh Narayan Lal wrote:

> Hi Peter,
>=20
> So based on the suggestions from you and Thomas, I think something like t=
he
> following should do the job within pci_alloc_irq_vectors_affinity():
>=20
> + =A0 =A0 =A0 if (!pci_is_managed(dev) && (hk_cpus < num_online_cpus()))
> + =A0 =A0 =A0 =A0 =A0 =A0 =A0 max_vecs =3D clamp(hk_cpus, min_vecs, max_v=
ecs);
>=20
> 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.