[PATCH v4] x86,cpu-hotplug: assign same CPU number to readded CPU

[Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>]

llc_shared_map is not cleared even if CPU is offline or hot removed.
So when hot-plugging CPU and assigning new CPU number to hot-added CPU,
the mask has wrong value. The mask is used by CSF schduler to create
sched_domain. So it breaks CFS scheduler.

Here is a example on my system.
My system has 4 sockets and each socket has 15 cores and HT is enabled.
In this case, each core of sockes is numbered as follows:

          | CPU#
Socket#0 | 0-14 , 60-74
Socket#1 | 15-29, 75-89
Socket#2 | 30-44, 90-104
Socket#3 | 45-59, 105-119

Then llc_shared_mask of CPU#30 has 0x3fff80000001fffc0000000.
It means that last level cache of Socket#2 is shared with
CPU#30-44 and 90-104.

When hot-removing socket#2 and #3, each core of sockets is numbered
as follows:

          | CPU#
Socket#0 | 0-14 , 60-74
Socket#1 | 15-29, 75-89

But llc_shared_mask is not cleared. So llc_shared_mask of CPU#30 remains
having 0x3fff80000001fffc0000000.

After that, when hot-adding socket#2 and #3, each core of sockets is
numbered as follows:

          | CPU#
Socket#0 | 0-14 , 60-74
Socket#1 | 15-29, 75-89
Socket#2 | 30-59
Socket#3 | 90-119

Then llc_shared_mask of CPU#30 becomes 0x3fff8000fffffffc0000000.
It means that last level cache of Socket#2 is shared with CPU#30-59
and 90-104. So the mask has wrong value.

At first, I cleared hot-removed CPU number's bit from llc_shared_map
when hot removing CPU. But Borislav suggested that the problem will
disappear if readded CPU is assigned same CPU number. And llc_shared_map
must not be changed.

So the patch assigns same CPU number to readded CPU by linking CPU
number to APIC ID. And by the patch, the problem disappers.

Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Suggested-by: Borislav Petkov <bp@alien8.de>
Reviewed-by: Toshi Kani <toshi.kani@hp.com>
---
v2: change cpuid to cpunum
v3: fix Borislav's email address of Suggested-by
     fix typo (ACPI ID to APIC ID)
v4: change cpu_used_xxx to cpu_number_xxx
---
  arch/x86/kernel/apic/apic.c | 33 ++++++++++++++++++++++++++++++++-
  1 file changed, 32 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index ad28db7..5cecc3b 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -220,6 +220,23 @@ static void apic_pm_activate(void);
  static unsigned long apic_phys;

  /*
+ * Bind APIC ID to Logical CPU number
+ * Logical CPU number to APIC ID does not change by this array
+ * even if CPU is hotplugged. So don't clear the array even if
+ * CPU is hot-removed
+ */
+static int apicid_to_cpunum[MAX_LOCAL_APIC] = {
+	[0 ... MAX_LOCAL_APIC-1] = -1,
+};
+
+/*
+ * Represent Logical CPU number bound to APIC ID
+ * Don't clear a bit even if CPU is hot-removed
+ */
+static DECLARE_BITMAP(cpu_number_bits, CONFIG_NR_CPUS);
+static struct cpumask *const cpu_number_mask = to_cpumask(cpu_number_bits);
+
+/*
   * Get the LAPIC version
   */
  static inline int lapic_get_version(void)
@@ -2122,6 +2139,17 @@ void disconnect_bsp_APIC(int virt_wire_setup)
  	apic_write(APIC_LVT1, value);
  }

+static int get_cpunum(int apicid)
+{
+	int cpu;
+
+	cpu = apicid_to_cpunum[apicid];
+	if (cpu < 0)
+		cpu = cpumask_next_zero(-1, cpu_number_mask);
+
+	return cpu;
+}
+
  int generic_processor_info(int apicid, int version)
  {
  	int cpu, max = nr_cpu_ids;
@@ -2199,7 +2227,9 @@ int generic_processor_info(int apicid, int version)
  		 */
  		cpu = 0;
  	} else
-		cpu = cpumask_next_zero(-1, cpu_present_mask);
+		cpu = get_cpunum(apicid);
+
+	apicid_to_cpunum[apicid] = cpu;

  	/*
  	 * Validate version
@@ -2228,6 +2258,7 @@ int generic_processor_info(int apicid, int version)
  	early_per_cpu(x86_cpu_to_logical_apicid, cpu) =
  		apic->x86_32_early_logical_apicid(cpu);
  #endif
+	cpumask_set_cpu(cpu, cpu_number_mask);
  	set_cpu_possible(cpu, true);
  	set_cpu_present(cpu, true);