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Subject: Re: [PATCH v3 4/4] PCI: Limit pci_alloc_irq_vectors() to housekeeping
 CPUs
To:     Bjorn Helgaas <helgaas@kernel.org>
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        linux-pci@vger.kernel.org, intel-wired-lan@lists.osuosl.org,
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References: <20200925202307.GA2456332@bjorn-Precision-5520>
From:   Nitesh Narayan Lal <nitesh@redhat.com>
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Message-ID: <256490f0-0762-447c-a7be-0e5a6bb04fc4@redhat.com>
Date:   Fri, 25 Sep 2020 17:38:05 -0400
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On 9/25/20 4:23 PM, Bjorn Helgaas wrote:
> On Fri, Sep 25, 2020 at 02:26:54PM -0400, Nitesh Narayan Lal wrote:
>> If we have isolated CPUs dedicated for use by real-time tasks, we try to
>> move IRQs to housekeeping CPUs from the userspace to reduce latency
>> overhead on the isolated CPUs.
>>
>> If we allocate too many IRQ vectors, moving them all to housekeeping CPU=
s
>> may exceed per-CPU vector limits.
>>
>> When we have isolated CPUs, limit the number of vectors allocated by
>> pci_alloc_irq_vectors() to the minimum number required by the driver, or
>> to one per housekeeping CPU if that is larger.
>>
>> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
>> ---
>>  include/linux/pci.h | 17 +++++++++++++++++
>>  1 file changed, 17 insertions(+)
>>
>> diff --git a/include/linux/pci.h b/include/linux/pci.h
>> index 835530605c0d..a7b10240b778 100644
>> --- a/include/linux/pci.h
>> +++ b/include/linux/pci.h
>> @@ -38,6 +38,7 @@
>>  #include <linux/interrupt.h>
>>  #include <linux/io.h>
>>  #include <linux/resource_ext.h>
>> +#include <linux/sched/isolation.h>
>>  #include <uapi/linux/pci.h>
>> =20
>>  #include <linux/pci_ids.h>
>> @@ -1797,6 +1798,22 @@ static inline int
>>  pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs,
>>  =09=09      unsigned int max_vecs, unsigned int flags)
>>  {
>> +=09unsigned int hk_cpus;
>> +
>> +=09hk_cpus =3D housekeeping_num_online_cpus(HK_FLAG_MANAGED_IRQ);
> Add blank line here before the block comment.
>
>> +=09/*
>> +=09 * If we have isolated CPUs for use by real-time tasks, to keep the
>> +=09 * latency overhead to a minimum, device-specific IRQ vectors are mo=
ved
>> +=09 * to the housekeeping CPUs from the userspace by changing their
>> +=09 * affinity mask. Limit the vector usage to keep housekeeping CPUs f=
rom
>> +=09 * running out of IRQ vectors.
>> +=09 */
>> +=09if (hk_cpus < num_online_cpus()) {
>> +=09=09if (hk_cpus < min_vecs)
>> +=09=09=09max_vecs =3D min_vecs;
>> +=09=09else if (hk_cpus < max_vecs)
>> +=09=09=09max_vecs =3D hk_cpus;
>> +=09}
> 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.

>>  =09return pci_alloc_irq_vectors_affinity(dev, min_vecs, max_vecs, flags=
,
>>  =09=09=09=09=09      NULL);
>>  }
>> --=20
>> 2.18.2
>>
--=20
Nitesh


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