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genirq: Get rid of global lock in irq_do_set_affinity()

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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 <jiangkunkun@huawei.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Kunkun Jiang <jiangkunkun@huawei.com>
Link: https://lore.kernel.org/all/20240826080618.3886694-1-maz@kernel.org
Link: https://lore.kernel.org/all/a7fc58e4-64c2-77fc-c1dc-f5eb78dbbb01@huawei.com
This commit is contained in:
Marc Zyngier 2024-08-26 09:06:18 +01:00 committed by Thomas Gleixner
parent 17e28a9aea
commit 64b6d1d7a8
1 changed files with 9 additions and 12 deletions
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21
kernel/irq/manage.c
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@ -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: