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

[Yasuaki Ishimatsu]

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@xxxxxxxxxxxxxx>
Suggested-by: Borislav Petkov <bp@xxxxxxxxxx>
---
 arch/x86/kernel/apic/apic.c | 32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index ad28db7..1cc715b 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -220,6 +220,22 @@ static void apic_pm_activate(void);
 static unsigned long apic_phys;

 /*
+ * Bind ACPI ID to CPU ID
+ * CPU ID 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_cpuid[MAX_LOCAL_APIC] = {
+	[0 ... MAX_LOCAL_APIC-1] = -1,
+};
+
+/*
+ * Represent CPU ID bound to APIC
+ * Don't clear a bit even if CPU is hot-removed
+ */
+static DECLARE_BITMAP(cpu_used_bits, CONFIG_NR_CPUS);
+static struct cpumask *const cpu_used_mask = to_cpumask(cpu_used_bits);
+
+/*
  * Get the LAPIC version
  */
 static inline int lapic_get_version(void)
@@ -2122,6 +2138,17 @@ void disconnect_bsp_APIC(int virt_wire_setup)
 	apic_write(APIC_LVT1, value);
 }

+static int get_cpuid(int apicid)
+{
+	int cpuid;
+
+	cpuid = apicid_to_cpuid[apicid];
+	if (cpuid < 0)
+		cpuid = cpumask_next_zero(-1, cpu_used_mask);
+
+	return cpuid;
+}
+
 int generic_processor_info(int apicid, int version)
 {
 	int cpu, max = nr_cpu_ids;
@@ -2199,7 +2226,9 @@ int generic_processor_info(int apicid, int version)
 		 */
 		cpu = 0;
 	} else
-		cpu = cpumask_next_zero(-1, cpu_present_mask);
+		cpu = get_cpuid(apicid);
+
+	apicid_to_cpuid[apicid] = cpu;

 	/*
 	 * Validate version
@@ -2228,6 +2257,7 @@ 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_used_mask);
 	set_cpu_possible(cpu, true);
 	set_cpu_present(cpu, true);


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