[RFC][Patch v1 2/3] i40e: limit msix vectors based on housekeeping CPUs

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

In a realtime environment, it is essential to isolate unwanted IRQs from
isolated CPUs to prevent latency overheads. Creating MSIX vectors only
based on the online CPUs could lead to a potential issue on an RT setup
that has several isolated CPUs but a very few housekeeping CPUs. This is
because in these kinds of setups an attempt to move the IRQs to the
limited housekeeping CPUs from isolated CPUs might fail due to the per
CPU vector limit. This could eventually result in latency spikes because
of the IRQ threads that we fail to move from isolated CPUs.

This patch prevents i40e to add vectors only based on available
housekeeping CPUs by using num_housekeeping_cpus().

Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
---
 drivers/net/ethernet/intel/i40e/i40e_main.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c b/drivers/net/ethernet/intel/i40e/i40e_main.c
index 2e433fdbf2c3..3b4cd4b3de85 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_main.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
@@ -5,6 +5,7 @@
 #include <linux/of_net.h>
 #include <linux/pci.h>
 #include <linux/bpf.h>
+#include <linux/sched/isolation.h>
 #include <generated/utsrelease.h>
 
 /* Local includes */
@@ -11002,7 +11003,7 @@ static int i40e_init_msix(struct i40e_pf *pf)
 	 * will use any remaining vectors to reach as close as we can to the
 	 * number of online CPUs.
 	 */
-	cpus = num_online_cpus();
+	cpus = num_housekeeping_cpus();
 	pf->num_lan_msix = min_t(int, cpus, vectors_left / 2);
 	vectors_left -= pf->num_lan_msix;
 
-- 
2.27.0