genirq: Get rid of global lock in irq_do_set_affinity()

Kunkun Jiang reports that for a workload involving the simultaneous startup
of a large number of VMs (for a total of about 200 vcpus), a lot of CPU
time gets spent on spinning on the tmp_mask_lock that exists as a static
raw spinlock in irq_do_set_affinity(). This lock protects a global cpumask
(tmp_mask) that is used as a temporary variable to compute the resulting
affinity.

While this is triggered by KVM issuing a irq_set_affinity() call each time
a vcpu is about to execute, it is obvious that having a single global
resource is not very scalable.

Since a cpumask can be a fairly large structure on systems with a high core
count, a stack allocation is not really appropriate.  Instead, turn the
global cpumask into a per-CPU variable, removing the need for locking
altogether as the code is executed with preemption and interrupts disabled.

[ tglx: Moved the per CPU variable declaration outside of the function ]

Reported-by: Kunkun Jiang <[email protected]>
Suggested-by: Thomas Gleixner <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Kunkun Jiang <[email protected]>
Link: https://lore.kernel.org/all/[email protected]
Link: https://lore.kernel.org/all/[email protected]

Author: Marc Zyngier

kernel/irq/manage.c

@@ -218,21 +218,20 @@ static void irq_validate_effective_affinity(struct irq_data *data)
 static inline void irq_validate_effective_affinity(struct irq_data *data) { }
 #endif
 
+static DEFINE_PER_CPU(struct cpumask, __tmp_mask);
+
 int irq_do_set_affinity(struct irq_data *data, const struct cpumask *mask,
 			bool force)
 {
+	struct cpumask *tmp_mask = this_cpu_ptr(&__tmp_mask);
 	struct irq_desc *desc = irq_data_to_desc(data);
 	struct irq_chip *chip = irq_data_get_irq_chip(data);
 	const struct cpumask  *prog_mask;
 	int ret;
 
-	static DEFINE_RAW_SPINLOCK(tmp_mask_lock);
-	static struct cpumask tmp_mask;
-
 	if (!chip || !chip->irq_set_affinity)
 		return -EINVAL;
 
-	raw_spin_lock(&tmp_mask_lock);
 	/*
 	 * If this is a managed interrupt and housekeeping is enabled on
 	 * it check whether the requested affinity mask intersects with
@@ -258,11 +257,11 @@ int irq_do_set_affinity(struct irq_data *data, const struct cpumask *mask,
 
 		hk_mask = housekeeping_cpumask(HK_TYPE_MANAGED_IRQ);
 
-		cpumask_and(&tmp_mask, mask, hk_mask);
-		if (!cpumask_intersects(&tmp_mask, cpu_online_mask))
+		cpumask_and(tmp_mask, mask, hk_mask);
+		if (!cpumask_intersects(tmp_mask, cpu_online_mask))
 			prog_mask = mask;
 		else
-			prog_mask = &tmp_mask;
+			prog_mask = tmp_mask;
 	} else {
 		prog_mask = mask;
 	}
@@ -272,16 +271,14 @@ int irq_do_set_affinity(struct irq_data *data, const struct cpumask *mask,
 	 * unless we are being asked to force the affinity (in which
 	 * case we do as we are told).
 	 */
-	cpumask_and(&tmp_mask, prog_mask, cpu_online_mask);
-	if (!force && !cpumask_empty(&tmp_mask))
-		ret = chip->irq_set_affinity(data, &tmp_mask, force);
+	cpumask_and(tmp_mask, prog_mask, cpu_online_mask);
+	if (!force && !cpumask_empty(tmp_mask))
+		ret = chip->irq_set_affinity(data, tmp_mask, force);
 	else if (force)
 		ret = chip->irq_set_affinity(data, mask, force);
 	else
 		ret = -EINVAL;
 
-	raw_spin_unlock(&tmp_mask_lock);
-
 	switch (ret) {
 	case IRQ_SET_MASK_OK:
 	case IRQ_SET_MASK_OK_DONE: