genirq, sched/isolation: Isolate from handling managed interrupts

The affinity of managed interrupts is completely handled in the kernel and
cannot be changed via the /proc/irq/* interfaces from user space. As the
kernel tries to spread out interrupts evenly accross CPUs on x86 to prevent
vector exhaustion, it can happen that a managed interrupt whose affinity
mask contains both isolated and housekeeping CPUs is routed to an isolated
CPU. As a consequence IO submitted on a housekeeping CPU causes interrupts
on the isolated CPU.

Add a new sub-parameter 'managed_irq' for 'isolcpus' and the corresponding
logic in the interrupt affinity selection code.

The subparameter indicates to the interrupt affinity selection logic that
it should try to avoid the above scenario.

This isolation is best effort and only effective if the automatically
assigned interrupt mask of a device queue contains isolated and
housekeeping CPUs. If housekeeping CPUs are online then such interrupts are
directed to the housekeeping CPU so that IO submitted on the housekeeping
CPU cannot disturb the isolated CPU.

If a queue's affinity mask contains only isolated CPUs then this parameter
has no effect on the interrupt routing decision, though interrupts are only
happening when tasks running on those isolated CPUs submit IO. IO submitted
on housekeeping CPUs has no influence on those queues.

If the affinity mask contains both housekeeping and isolated CPUs, but none
of the contained housekeeping CPUs is online, then the interrupt is also
routed to an isolated CPU. Interrupts are only delivered when one of the
isolated CPUs in the affinity mask submits IO. If one of the contained
housekeeping CPUs comes online, the CPU hotplug logic migrates the
interrupt automatically back to the upcoming housekeeping CPU. Depending on
the type of interrupt controller, this can require that at least one
interrupt is delivered to the isolated CPU in order to complete the
migration.

[ tglx: Removed unused parameter, added and edited comments/documentation
  	and rephrased the changelog so it contains more details. ]

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200120091625.17912-1-ming.lei@redhat.com

1 files changed, 19 insertions, 2 deletions

diff --git a/kernel/irq/cpuhotplug.c b/kernel/irq/cpuhotplug.c
index 6c7ca2e983a5..02236b13b359 100644
--- a/kernel/irq/cpuhotplug.c
+++ b/kernel/irq/cpuhotplug.c
@@ -12,6 +12,7 @@
 #include <linux/interrupt.h>
 #include <linux/ratelimit.h>
 #include <linux/irq.h>
+#include <linux/sched/isolation.h>
 
 #include "internals.h"
 
@@ -171,6 +172,20 @@ void irq_migrate_all_off_this_cpu(void)
 	}
 }
 
+static bool hk_should_isolate(struct irq_data *data, unsigned int cpu)
+{
+	const struct cpumask *hk_mask;
+
+	if (!housekeeping_enabled(HK_FLAG_MANAGED_IRQ))
+		return false;
+
+	hk_mask = housekeeping_cpumask(HK_FLAG_MANAGED_IRQ);
+	if (cpumask_subset(irq_data_get_effective_affinity_mask(data), hk_mask))
+		return false;
+
+	return cpumask_test_cpu(cpu, hk_mask);
+}
+
 static void irq_restore_affinity_of_irq(struct irq_desc *desc, unsigned int cpu)
 {
 	struct irq_data *data = irq_desc_get_irq_data(desc);
@@ -188,9 +203,11 @@ static void irq_restore_affinity_of_irq(struct irq_desc *desc, unsigned int cpu)
 	/*
 	 * If the interrupt can only be directed to a single target
 	 * CPU then it is already assigned to a CPU in the affinity
-	 * mask. No point in trying to move it around.
+	 * mask. No point in trying to move it around unless the
+	 * isolation mechanism requests to move it to an upcoming
+	 * housekeeping CPU.
 	 */
-	if (!irqd_is_single_target(data))
+	if (!irqd_is_single_target(data) || hk_should_isolate(data, cpu))
 		irq_set_affinity_locked(data, affinity, false);
 }