[PATCH v3 3/3] x86, x2apic: use multiple cluster members for the irq destination only with the explicit affinity

[Suresh Siddha]

During boot or driver load etc, interrupt destination is setup using default
target cpu's. Later the user (irqbalance etc) or the driver (irq_set_affinity/
irq_set_affinity_hint) can request the interrupt to be migrated to some
specific set of cpu's.

In the x2apic cluster routing, for the default scenario use single cpu as the
interrupt destination and when there is an explicit interrupt affinity
request, route the interrupt to multiple members of a x2apic cluster
specified in the cpumask of the migration request.

This will minmize the vector pressure when there are lot of interrupt
sources and relatively few x2apic clusters (for example a single socket
server). This will allow the performance critical interrupts to be
routed to multiple cpu's in the x2apic cluster (irqbalance for example
uses the cache siblings etc while specifying the interrupt destination) and
allow non-critical interrupts to be serviced by a single logical cpu.

Signed-off-by: Suresh Siddha <suresh.b.siddha@xxxxxxxxx>
Acked-by: Yinghai Lu <yinghai@xxxxxxxxxx>
Acked-by: Alexander Gordeev <agordeev@xxxxxxxxxx>
Acked-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
---
 arch/x86/kernel/apic/x2apic_cluster.c |   21 +++++++++++++++++++--
 1 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/apic/x2apic_cluster.c b/arch/x86/kernel/apic/x2apic_cluster.c
index bde78d0..c88baa4 100644
--- a/arch/x86/kernel/apic/x2apic_cluster.c
+++ b/arch/x86/kernel/apic/x2apic_cluster.c
@@ -209,13 +209,30 @@ static int x2apic_cluster_probe(void)
 		return 0;
 }
 
+static const struct cpumask *x2apic_cluster_target_cpus(void)
+{
+	return cpu_all_mask;
+}
+
 /*
  * Each x2apic cluster is an allocation domain.
  */
 static void cluster_vector_allocation_domain(int cpu, struct cpumask *retmask,
 					     const struct cpumask *mask)
 {
-	cpumask_and(retmask, mask, per_cpu(cpus_in_cluster, cpu));
+	/*
+	 * To minimize vector pressure, default case of boot, device bringup
+	 * etc will use a single cpu for the interrupt destination.
+	 *
+	 * On explicit migration requests coming from irqbalance etc,
+	 * interrupts will be routed to the x2apic cluster (cluster-id
+	 * derived from the first cpu in the mask) members specified
+	 * in the mask.
+	 */
+	if (mask == x2apic_cluster_target_cpus())
+		cpumask_copy(retmask, cpumask_of(cpu));
+	else
+		cpumask_and(retmask, mask, per_cpu(cpus_in_cluster, cpu));
 }
 
 static struct apic apic_x2apic_cluster = {
@@ -229,7 +246,7 @@ static struct apic apic_x2apic_cluster = {
 	.irq_delivery_mode		= dest_LowestPrio,
 	.irq_dest_mode			= 1, /* logical */
 
-	.target_cpus			= online_target_cpus,
+	.target_cpus			= x2apic_cluster_target_cpus,
 	.disable_esr			= 0,
 	.dest_logical			= APIC_DEST_LOGICAL,
 	.check_apicid_used		= NULL,
-- 
1.7.6.5

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