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Subject: Re: [PATCH v4 4/4] PCI: Limit pci_alloc_irq_vectors() to housekeeping
 CPUs
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        linux-pci@vger.kernel.org, intel-wired-lan@lists.osuosl.org,
        frederic@kernel.org, mtosatti@redhat.com, sassmann@redhat.com,
        jesse.brandeburg@intel.com, lihong.yang@intel.com,
        helgaas@kernel.org, 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
References: <20200928183529.471328-1-nitesh@redhat.com>
 <20200928183529.471328-5-nitesh@redhat.com>
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From:   Nitesh Narayan Lal <nitesh@redhat.com>
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Message-ID: <79f382a7-883d-ff42-394d-ec4ce81fed6a@redhat.com>
Date:   Sun, 18 Oct 2020 14:14:46 -0400
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On 10/16/20 8:20 AM, Peter Zijlstra wrote:
> On Mon, Sep 28, 2020 at 02:35:29PM -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>
>> ---
>>  drivers/pci/msi.c | 18 ++++++++++++++++++
>>  1 file changed, 18 insertions(+)
>>
>> diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
>> index 30ae4ffda5c1..8c156867803c 100644
>> --- a/drivers/pci/msi.c
>> +++ b/drivers/pci/msi.c
>> @@ -23,6 +23,7 @@
>>  #include <linux/slab.h>
>>  #include <linux/irqdomain.h>
>>  #include <linux/of_irq.h>
>> +#include <linux/sched/isolation.h>
>> =20
>>  #include "pci.h"
>> =20
>> @@ -1191,8 +1192,25 @@ int pci_alloc_irq_vectors_affinity(struct pci_dev=
 *dev, unsigned int min_vecs,
>>  =09=09=09=09   struct irq_affinity *affd)
>>  {
>>  =09struct irq_affinity msi_default_affd =3D {0};
>> +=09unsigned int hk_cpus;
>>  =09int nvecs =3D -ENOSPC;
>> =20
>> +=09hk_cpus =3D housekeeping_num_online_cpus(HK_FLAG_MANAGED_IRQ);
>> +
>> +=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;
> is that:
>
> =09=09max_vecs =3D clamp(hk_cpus, min_vecs, max_vecs);

Yes, I think this will do.

>
> 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.

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() +
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0 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?

>
> And what are the (desired) semantics vs hotplug? Using a cpumask without
> excluding hotplug is racy.

The housekeeping_mask should still remain constant, isn't?
In any case, I can double check this.

>
>> +=09}
>> +
>>  =09if (flags & PCI_IRQ_AFFINITY) {
>>  =09=09if (!affd)
>>  =09=09=09affd =3D &msi_default_affd;
>> --=20
>> 2.18.2
>>
--=20
Thanks
Nitesh


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