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SubjectRe: [PATCH v4 4/4] PCI: Limit pci_alloc_irq_vectors() to housekeeping CPUs
[to: Christoph in case he has comments, since I think he wrote this code]

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 CPUs
> 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>

Acked-by: Bjorn Helgaas <bhelgaas@google.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>
>
> #include "pci.h"
>
> @@ -1191,8 +1192,25 @@ int pci_alloc_irq_vectors_affinity(struct pci_dev *dev, unsigned int min_vecs,
> struct irq_affinity *affd)
> {
> struct irq_affinity msi_default_affd = {0};
> + unsigned int hk_cpus;
> int nvecs = -ENOSPC;
>
> + hk_cpus = housekeeping_num_online_cpus(HK_FLAG_MANAGED_IRQ);
> +
> + /*
> + * If we have isolated CPUs for use by real-time tasks, to keep the
> + * latency overhead to a minimum, device-specific IRQ vectors are moved
> + * to the housekeeping CPUs from the userspace by changing their
> + * affinity mask. Limit the vector usage to keep housekeeping CPUs from
> + * running out of IRQ vectors.
> + */
> + if (hk_cpus < num_online_cpus()) {
> + if (hk_cpus < min_vecs)
> + max_vecs = min_vecs;
> + else if (hk_cpus < max_vecs)
> + max_vecs = hk_cpus;
> + }
> +
> if (flags & PCI_IRQ_AFFINITY) {
> if (!affd)
> affd = &msi_default_affd;
> --
> 2.18.2
>

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