[PATCH v3] Drivers: hv: vmbus: Optimize boot time by concurrent execution of hv_synic_init()

[Saurabh Sengar <ssengar@linux.microsoft.com>]

Currently on a very large system with 1780 CPUs, hv_acpi_init() takes
around 3 seconds to complete. This is because of sequential synic
initialization for each CPU performed by hv_synic_init().

Schedule these tasks parallelly so that each CPU executes hv_synic_init()
in parallel to take full advantage of multiple CPUs.

This solution saves around 2 seconds of boot time on a 1780 CPU system,
which is around 66% improvement in the existing logic.

Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
Reviewed-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
Reviewed-by: Srivatsa S. Bhat (Microsoft) <srivatsa@csail.mit.edu>
Reviewed-by: Dexuan Cui <decui@microsoft.com>
---
[V3]
 - Modify comment, removing hotplug mention
 - Added Reviewed-by

[V2]
 - used cpuhp_setup_state_nocalls_cpuslocked instead of internal function
 - improve commit message and subject
 - Added a comment for cpu hotplug callback

 drivers/hv/vmbus_drv.c | 34 +++++++++++++++++++++++++++++++---
 1 file changed, 31 insertions(+), 3 deletions(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index c857dc3975be..7242c4920427 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1306,6 +1306,13 @@ static irqreturn_t vmbus_percpu_isr(int irq, void *dev_id)
 	return IRQ_HANDLED;
 }
 
+static void vmbus_percpu_work(struct work_struct *work)
+{
+	unsigned int cpu = smp_processor_id();
+
+	hv_synic_init(cpu);
+}
+
 /*
  * vmbus_bus_init -Main vmbus driver initialization routine.
  *
@@ -1316,7 +1323,8 @@ static irqreturn_t vmbus_percpu_isr(int irq, void *dev_id)
  */
 static int vmbus_bus_init(void)
 {
-	int ret;
+	int ret, cpu;
+	struct work_struct __percpu *works;
 
 	ret = hv_init();
 	if (ret != 0) {
@@ -1355,12 +1363,32 @@ static int vmbus_bus_init(void)
 	if (ret)
 		goto err_alloc;
 
+	works = alloc_percpu(struct work_struct);
+	if (!works) {
+		ret = -ENOMEM;
+		goto err_alloc;
+	}
+
 	/*
 	 * Initialize the per-cpu interrupt state and stimer state.
 	 * Then connect to the host.
 	 */
-	ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "hyperv/vmbus:online",
-				hv_synic_init, hv_synic_cleanup);
+	cpus_read_lock();
+	for_each_online_cpu(cpu) {
+		struct work_struct *work = per_cpu_ptr(works, cpu);
+
+		INIT_WORK(work, vmbus_percpu_work);
+		schedule_work_on(cpu, work);
+	}
+
+	for_each_online_cpu(cpu)
+		flush_work(per_cpu_ptr(works, cpu));
+
+	/* Register the callbacks for possible CPU online/offline'ing */
+	ret = cpuhp_setup_state_nocalls_cpuslocked(CPUHP_AP_ONLINE_DYN, "hyperv/vmbus:online",
+						   hv_synic_init, hv_synic_cleanup);
+	cpus_read_unlock();
+	free_percpu(works);
 	if (ret < 0)
 		goto err_alloc;
 	hyperv_cpuhp_online = ret;
-- 
2.43.0