Re: [PATCH v3] x86,cpu-hotplug: assign same CPU number to readded CPU
From: Yasuaki Ishimatsu
Date: Tue Jul 22 2014 - 22:29:54 EST
(2014/07/23 3:29), Toshi Kani wrote:
On Thu, 2014-07-17 at 16:32 +0900, Yasuaki Ishimatsu wrote:
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@xxxxxxxxx>
---
v2: change cpuid to cpunum
v3: fix Borislav's email address of Suggested-by
fix typo (ACPI ID to APIC ID)
---
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..5dc3e50 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_used_bits, CONFIG_NR_CPUS);
+static struct cpumask *const cpu_used_mask = to_cpumask(cpu_used_bits);
I think the name of cpu_used_mask is confusing since the term "used"
sounds some state of CPU, which is not the case here. How about
cpu_number_mask? This is more specific and avoids such confusion.
I'll update it soon.
Otherwise, the change looks good.
Reviewed-by: Toshi Kani <toshi.kani@xxxxxx>
Thank you for review.
Thanks,
Yasuaki Ishimatsu
Thanks,
-Toshi
+
+/*
* Get the LAPIC version
*/
^^
ps. I had a problem in applying this patch since there was an extra
blank character at the begging of unchanged lines. But this might be an
issue in my mailer.
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