Re: [PATCH] genirq: set initial affinity when hinting

[Jesse Brandeburg <jesse.brandeburg@gmail.com>]

+netdev, as network driver developers might be interested in this functionality.

On Thu, Dec 18, 2014 at 5:22 PM, Jesse Brandeburg
<jesse.brandeburg@intel.com> wrote:
> Problem:
> The default behavior of the kernel is somewhat undesirable as all
> requested interrupts end up on CPU0 after registration.  A user can
> run irqbalance daemon, or can manually configure smp_affinity via the
> proc filesystem, but the default affinity of the interrupts for all
> devices is always CPU zero, this can cause performance problems or
> very heavy cpu use of only one core if not noticed and fixed by the
> user.
>
> Patch:
> This patch enables the setting of the initial affinity directly
> when the driver sets a hint.
>
> This enabling means that kernel drivers can include an initial
> affinity setting for the interrupt, instead of all interrupts starting
> out life on CPU0. Of course if irqbalance is still running then the
> interrupts will get moved as before.
>
> This function is currently called by drivers in block, crypto,
> infiniband, ethernet and scsi trees, but only a handful, so these will
> be the devices affected by this change.
>
> Tested on i40e, and default interrupts were spread across the CPUs
> according to the hint.
>
> drivers/block/mtip32xx/mtip32xx.c:3
> drivers/block/nvme-core.c:2
> drivers/crypto/qat/qat_dh895xcc/adf_isr.c:3
> drivers/infiniband/hw/qib/qib_iba7322.c:2
> drivers/net/ethernet/intel/i40e/i40e_main.c:3
> drivers/net/ethernet/intel/i40evf/i40evf_main.c:3
> drivers/net/ethernet/intel/ixgbe/ixgbe_main.c:3
> drivers/net/ethernet/mellanox/mlx4/en_cq.c:2
> drivers/scsi/hpsa.c:3
> drivers/scsi/lpfc/lpfc_init.c:3
> drivers/scsi/megaraid/megaraid_sas_base.c:8
> drivers/soc/ti/knav_qmss_acc.c:1
> drivers/soc/ti/knav_qmss_queue.c:2
> drivers/virtio/virtio_pci_common.c:2
>
> Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
> CC: Thomas Gleixner <tglx@linutronix.de>
> ---
>  kernel/irq/manage.c |    2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
> index 8069237..f038e58 100644
> --- a/kernel/irq/manage.c
> +++ b/kernel/irq/manage.c
> @@ -243,6 +243,8 @@ int irq_set_affinity_hint(unsigned int irq, const struct cpumask *m)
>                 return -EINVAL;
>         desc->affinity_hint = m;
>         irq_put_desc_unlock(desc, flags);
> +       /* set the initial affinity to prevent every interrupt being on CPU0 */
> +       __irq_set_affinity(irq, m, false);
>         return 0;
>  }
>  EXPORT_SYMBOL_GPL(irq_set_affinity_hint);
>