[PATCH 0/2] Add CONFIG_PERMANENT_CPU_TOPOLOGY

[Prarit Bhargava <prarit@redhat.com>]

The information in /sys/devices/system/cpu/cpuX/topology
directory is useful for userspace monitoring applications and in-tree
utilities like cpupower & turbostat.

When down'ing a CPU the /sys/devices/system/cpu/cpuX/topology directory is
removed during the CPU_DEAD hotplug callback in the kernel.  The problem
with this model is that the CPU has not been physically removed and the
data in the topology directory is still valid.

This patchset adds CONFIG_PERMANENT_CPU_TOPOLOGY, and is Y by default for
x86, an N for all other arches.  When enabled the kernel is modified so
that the topology directory is added to the core cpu sysfs files so that
the topology directory exists for the lifetime of the CPU.  When
disabled, the behavior of the current kernel is maintained (that is, the
topology directory is removed on a down and added on an up).  Adding
CONFIG_PERMANENT_CPU_TOPOLOGY may require additional architecture so that
the cpumask data the CPU's topology is not cleared during a CPU down.

This patchset combines drivers/base/topology.c and drivers/base/cpu.c to
implement CONFIG_PERMANENT_CPU_TOPOLOGY, and leaves all arches except
x86 with the current behavior, and adds an additional minor cleanup to
drivers/base/cpu.c.

Before patch:

[root@prarit cpu143]# ls
cache    crash_notes       firmware_node  online     thermal_throttle
cpufreq  crash_notes_size  microcode      power      topology
cpuidle  driver            node3          subsystem  uevent

Down a cpu

[root@prarit cpu143]# echo 0 > online

[root@intel-brickland-05 cpu143]# ls
cpuidle      crash_notes_size  firmware_node  online  subsystem
crash_notes  driver            node3          power   uevent
[root@intel-brickland-05 cpu143]# ls -l topology
ls: cannot access topology: No such file or directory

After patch:

[root@prarit cpu143]# ls
cache    crash_notes       firmware_node  online     thermal_throttle
cpufreq  crash_notes_size  microcode      power      topology
cpuidle  driver            node3          subsystem  uevent
[root@prarit cpu143]# cat topology/*
27
ffff,c0000000,000000ff,ffc00000,00000000
54-71,126-143
3
8000,00000000,00000080,00000000,00000000
71,143

Down a cpu

[root@prarit cpu143]# echo 0 > online
[root@intel-brickland-05 cpu143]# ls
cpuidle      crash_notes_size  firmware_node  online  subsystem  uevent
crash_notes  driver            node3          power   topology
[root@prarit cpu143]# cat topology/*
27
ffff,c0000000,000000ff,ffc00000,00000000
54-71,126-143
3
8000,00000000,00000080,00000000,00000000
71,143

I did some light testing with and without BOOTPARAM_HOTPLUG_CPU0 enabled,
and up'd and down'd CPUs in sequence, randomly, by thread group, by
socket group and didn't see any issues.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Len Brown <len.brown@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Zhu Guihua <zhugh.fnst@cn.fujitsu.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: "Jan H. Schönherr" <jschoenh@amazon.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Thomas Renninger <trenn@suse.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Prarit Bhargava <prarit@redhat.com>

Prarit Bhargava (2):
  cpu hotplug, add CONFIG_PERMANENT_CPU_TOPOLOGY and keep topology
    directory for lifetime of CPU
  base, cpu, remove hotplugable_cpu_attr_groups

 arch/x86/kernel/smpboot.c |   28 --------
 drivers/base/Kconfig      |   13 ++++
 drivers/base/Makefile     |    2 +-
 drivers/base/cpu.c        |  141 ++++++++++++++++++++++++++++++++++++++---
 drivers/base/topology.c   |  155 ---------------------------------------------
 5 files changed, 146 insertions(+), 193 deletions(-)
 delete mode 100644 drivers/base/topology.c

-- 
1.7.9.3