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]

0 files changed, 0 insertions, 0 deletions