docs: add io_queue flag to isolcpus

The io_queue flag informs multiqueue device drivers where to place
hardware queues. Document this new flag in the isolcpus
command-line argument description.

Reviewed-by: Aaron Tomlin <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Signed-off-by: Daniel Wagner <[email protected]>
Signed-off-by: Aaron Tomlin <[email protected]>

Author: igaw

Documentation/admin-guide/kernel-parameters.txt

@@ -2816,7 +2816,6 @@ Kernel parameters
 			  "number of CPUs in system - 1".
 
 			managed_irq
-
 			  Isolate from being targeted by managed interrupts
 			  which have an interrupt mask containing isolated
 			  CPUs. The affinity of managed interrupts is
@@ -2839,6 +2838,27 @@ Kernel parameters
 			  housekeeping CPUs has no influence on those
 			  queues.
 
+			io_queue
+			  Isolate from I/O queue work caused by multiqueue
+			  device drivers. Restrict the placement of
+			  queues to housekeeping CPUs only, ensuring that
+			  all I/O work is processed by a housekeeping CPU.
+
+			  The io_queue configuration takes precedence
+			  over managed_irq. When io_queue is used,
+			  managed_irq placement constrains have no
+			  effect.
+
+			  Note: Offlining housekeeping CPUS which serve
+			  isolated CPUs will be rejected. Isolated CPUs
+			  need to be offlined before offlining the
+			  housekeeping CPUs.
+
+			  Note: When an isolated CPU issues an I/O request,
+			  it is forwarded to a housekeeping CPU. This will
+			  trigger a software interrupt on the completion
+			  path.
+
 			The format of <cpu-list> is described above.
 
 	iucv=		[HW,NET]