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.

Signed-off-by: Daniel Wagner <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
[atomlin: Refined io_queue kernel parameter documentation]
Signed-off-by: Aaron Tomlin <[email protected]>

Author: igaw

Documentation/admin-guide/kernel-parameters.txt

@@ -2810,7 +2810,6 @@ 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
@@ -2833,6 +2832,35 @@ Kernel parameters
 			  housekeeping CPUs has no influence on those
 			  queues.
 
+			io_queue
+			  Applicable to managed IRQs only. Restrict
+			  multiqueue hardware queue allocation to online
+			  housekeeping CPUs. This guarantees that all
+			  managed hardware completion interrupts are routed
+			  exclusively to housekeeping cores, shielding
+			  isolated CPUs from I/O interruptions even if they
+			  initiated the request.
+
+			  The io_queue configuration takes precedence over
+			  managed_irq. When io_queue is used, managed_irq
+			  placement constraints have no effect.
+
+			  Note: Using io_queue restricts the number of
+			  allocated hardware queues to match the number of
+			  housekeeping CPUs. This prevents MSI-X vector
+			  exhaustion and forces isolated CPUs to share
+			  submission queues.
+
+			  Note: Offlining housekeeping CPUs which serve
+			  isolated CPUs will fail. The isolated CPUs must
+			  be offlined before offlining the housekeeping
+			  CPUs.
+
+			  Note: When I/O is submitted by an application on
+			  an isolated CPU, the hardware completion
+			  interrupt is handled entirely by a housekeeping
+			  CPU.
+
 			The format of <cpu-list> is described above.
 
 	iucv=		[HW,NET]