From fdba46ffb4c203b6e6794163493fd310f98bb4be Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 13 Sep 2017 23:29:27 +0200
Subject: [PATCH] x86/apic: Get rid of multi CPU affinity

Setting the interrupt affinity of a single interrupt to multiple CPUs has a
dubious value.

 1) This only works on machines where the APIC uses logical destination
    mode. If the APIC uses physical destination mode then it is already
    restricted to a single CPU

 2) Experiments have shown, that the benefit of multi CPU affinity is close
    to zero and in some test even worse than setting the affinity to a
    single CPU.

    The reason for this is that the delivery targets the APIC with the
    lowest ID first and only if that APIC is busy (servicing an interrupt,
    i.e. ISR is not empty) it hands it over to the next APIC. In the
    conducted tests the vast majority of interrupts ends up on the APIC
    with the lowest ID anyway, so there is no natural spreading of the
    interrupts possible.

Supporting multi CPU affinities adds a lot of complexity to the code, which
can turn the allocation search into a worst case of

    nr_vectors * nr_online_cpus * nr_bits_in_target_mask

As a first step disable it by restricting the vector search to a single
CPU.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Juergen Gross <jgross@suse.com>
Tested-by: Yu Chen <yu.c.chen@intel.com>
Acked-by: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Alok Kataria <akataria@vmware.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Rui Zhang <rui.zhang@intel.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Len Brown <lenb@kernel.org>
Link: https://lkml.kernel.org/r/20170913213154.228824430@linutronix.de
---
 arch/x86/kernel/apic/vector.c | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kernel/apic/vector.c b/arch/x86/kernel/apic/vector.c
index 67d20ee60e33..93edc2236282 100644
--- a/arch/x86/kernel/apic/vector.c
+++ b/arch/x86/kernel/apic/vector.c
@@ -136,8 +136,7 @@ static int __assign_irq_vector(int irq, struct apic_chip_data *d,
 	while (cpu < nr_cpu_ids) {
 		int new_cpu, offset;
 
-		/* Get the possible target cpus for @mask/@cpu from the apic */
-		apic->vector_allocation_domain(cpu, vector_cpumask, mask);
+		cpumask_copy(vector_cpumask, cpumask_of(cpu));
 
 		/*
 		 * Clear the offline cpus from @vector_cpumask for searching
@@ -367,17 +366,11 @@ static int x86_vector_alloc_irqs(struct irq_domain *domain, unsigned int virq,
 		irq_data->chip = &lapic_controller;
 		irq_data->chip_data = data;
 		irq_data->hwirq = virq + i;
+		irqd_set_single_target(irq_data);
 		err = assign_irq_vector_policy(virq + i, node, data, info,
 					       irq_data);
 		if (err)
 			goto error;
-		/*
-		 * If the apic destination mode is physical, then the
-		 * effective affinity is restricted to a single target
-		 * CPU. Mark the interrupt accordingly.
-		 */
-		if (!apic->irq_dest_mode)
-			irqd_set_single_target(irq_data);
 	}
 
 	return 0;
@@ -434,7 +427,7 @@ static void __init init_legacy_irqs(void)
 		BUG_ON(!data);
 
 		data->cfg.vector = ISA_IRQ_VECTOR(i);
-		cpumask_setall(data->domain);
+		cpumask_copy(data->domain, cpumask_of(0));
 		irq_set_chip_data(i, data);
 	}
 }
-- 
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