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

[Nitesh Narayan Lal <nitesh@redhat.com>]

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 <nitesh@redhat.com>
---
 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);
 }
-- 
2.27.0