Subject: Re: [RFC][Patch v1 3/3] PCI: Limit pci_alloc_irq_vectors as per housekeeping CPUs

From: Marcelo Tosatti <[email-masked]>
Date: Thu, 10 Sep 2020 16:22:08 -0300

On Wed, Sep 09, 2020 at 11:08:18AM -0400, Nitesh Narayan Lal wrote:
> This patch limits the pci_alloc_irq_vectors max vectors that is passed on
> by the caller based on the available housekeeping CPUs by only using the
> minimum of the two.
> 
> A minimum of the max_vecs passed and available housekeeping CPUs is
> derived to ensure that we don't create excess vectors which can be
> problematic specifically in an RT environment. This is because for an RT
> environment unwanted IRQs are moved to the housekeeping CPUs from
> isolated CPUs to keep the latency overhead to a minimum. If the number of
> housekeeping CPUs are significantly lower than that of the isolated CPUs
> we can run into failures while moving these IRQs to housekeeping due to
> per CPU vector limit.
> 
> Signed-off-by: Nitesh Narayan Lal <[email-masked]>
> ---
>  include/linux/pci.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 835530605c0d..750ba927d963 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 +1798,21 @@ static inline int
>  pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs,
>  		      unsigned int max_vecs, unsigned int flags)
>  {
> +	unsigned int num_housekeeping = num_housekeeping_cpus();
> +	unsigned int num_online = num_online_cpus();
> +
> +	/*
> +	 * Try to be conservative and at max only ask for the same number of
> +	 * vectors as there are housekeeping CPUs. However, skip any
> +	 * modification to the of max vectors in two conditions:
> +	 * 1. If the min_vecs requested are higher than that of the
> +	 *    housekeeping CPUs as we don't want to prevent the initialization
> +	 *    of a device.
> +	 * 2. If there are no isolated CPUs as in this case the driver should
> +	 *    already have taken online CPUs into consideration.
> +	 */
> +	if (min_vecs < num_housekeeping && num_housekeeping != num_online)
> +		max_vecs = min_t(int, max_vecs, num_housekeeping);
>  	return pci_alloc_irq_vectors_affinity(dev, min_vecs, max_vecs, flags,
>  					      NULL);
>  }

If min_vecs > num_housekeeping, for example:

/* PCI MSI/MSIx support */
#define XGBE_MSI_BASE_COUNT     4
#define XGBE_MSI_MIN_COUNT      (XGBE_MSI_BASE_COUNT + 1)

Then the protection fails.

How about reducing max_vecs down to min_vecs, if min_vecs >
num_housekeeping ?