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 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@xxxxxxxxxx>
> ---
> 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>
>
> #include <linux/pci_ids.h>
> @@ -1797,6 +1798,22 @@ static inline int
> pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs,
> unsigned int max_vecs, unsigned int flags)
> {
> + unsigned int hk_cpus;
> +
> + hk_cpus = housekeeping_num_online_cpus(HK_FLAG_MANAGED_IRQ);
Add blank line here before the block comment.
> + /*
> + * 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;
> + }
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.
> return pci_alloc_irq_vectors_affinity(dev, min_vecs, max_vecs, flags,
> NULL);
> }
> --
> 2.18.2
>
