[PATCH v5] x86,cpu-hotplug: assign same CPU number to readded CPU

Yasuaki Ishimatsu
2014-09-04T02:48:03Z

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
v5: rebase to 3.17-rc3
---
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 6776027..c476827 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)
@@ -2109,6 +2126,17 @@ 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;
@@ -2186,7 +2214,9 @@ 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
@@ -2215,6 +2245,7 @@ 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);

--
1.8.3.1