[RFC PATCH v2] ACPI / processor: Fix possible CPUs map

[Dou Liyang <douly.fnst@cn.fujitsu.com>]

Rafael J told me in order for the ACPI-based physical CPU hotplug to work,
there have to be objects in the ACPI namespace corresponding to all of the
processors in question. If they are not present, there is no way to signal
insertion and eject the processors safely.

But, Kernel calculates the possible CPU count from the number of Local APIC
entries in ACPI MADT. It doesn't consider with the ACPI namespace and
reports unrealistically high numbers. And kernel allocates resources
according to num_possible_cpus(), such as vectors, that may cause vector
space exhaustion and even bugs.

Depth-first search the namespace tree, check and collect the correct CPUs
and update the possible map.

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
---
Changelog v1 --> v2:
 -Optimize the code by Andy Shevchenko's suggestion
 -modify the changelog

 drivers/acpi/acpi_processor.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c
index 449d86d39965..ac45380f4439 100644
--- a/drivers/acpi/acpi_processor.c
+++ b/drivers/acpi/acpi_processor.c
@@ -671,6 +671,24 @@ static acpi_status __init acpi_processor_ids_walk(acpi_handle handle,
 
 }
 
+static void __init acpi_update_possible_map(void)
+{
+	unsigned int cpu;
+
+	if (nr_unique_ids >= nr_cpu_ids)
+		return;
+
+	/* Don't yet figure out if it's superfluous */
+	if (nr_unique_ids >= cpumask_last(cpu_possible_mask))
+		return;
+
+	for_each_cpu_wrap(cpu, cpu_possible_mask, nr_unique_ids)
+		set_cpu_possible(cpu, false);
+
+	nr_cpu_ids = nr_unique_ids;
+	pr_info("Allowing %d possible CPUs\n", nr_cpu_ids);
+}
+
 static void __init acpi_processor_check_duplicates(void)
 {
 	/* check the correctness for all processors in ACPI namespace */
@@ -680,6 +698,9 @@ static void __init acpi_processor_check_duplicates(void)
 						NULL, NULL, NULL);
 	acpi_get_devices(ACPI_PROCESSOR_DEVICE_HID, acpi_processor_ids_walk,
 						NULL, NULL);
+
+	/* make possible CPU count more realistic */
+	acpi_update_possible_map();
 }
 
 bool acpi_duplicate_processor_id(int proc_id)
-- 
2.14.3