Re: [PATCH 1/2] cpu hotplug, add CONFIG_PERMANENT_CPU_TOPOLOGY and keep topology directory for lifetime of CPU

[Prarit Bhargava <prarit@redhat.com>]

On 10/19/2015 11:04 AM, Prarit Bhargava wrote:
> 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 patch 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 patch 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.
> 
> 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>
> Signed-off-by: Prarit Bhargava <prarit@redhat.com>
> ---
>  arch/x86/kernel/smpboot.c |   28 --------
>  drivers/base/Kconfig      |   13 ++++
>  drivers/base/Makefile     |    2 +-
>  drivers/base/cpu.c        |  135 +++++++++++++++++++++++++++++++++++++++
>  drivers/base/topology.c   |  155 ---------------------------------------------
>  5 files changed, 149 insertions(+), 184 deletions(-)
>  delete mode 100644 drivers/base/topology.c
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index e0c198e..19082c7 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1322,32 +1322,6 @@ __init void prefill_possible_map(void)
>  
>  #ifdef CONFIG_HOTPLUG_CPU
>  
> -static void remove_siblinginfo(int cpu)
> -{
> -	int sibling;
> -	struct cpuinfo_x86 *c = &cpu_data(cpu);
> -
> -	for_each_cpu(sibling, topology_core_cpumask(cpu)) {
> -		cpumask_clear_cpu(cpu, topology_core_cpumask(sibling));
> -		/*/
> -		 * last thread sibling in this cpu core going down
> -		 */
> -		if (cpumask_weight(topology_sibling_cpumask(cpu)) == 1)
> -			cpu_data(sibling).booted_cores--;
> -	}
> -
> -	for_each_cpu(sibling, topology_sibling_cpumask(cpu))
> -		cpumask_clear_cpu(cpu, topology_sibling_cpumask(sibling));
> -	for_each_cpu(sibling, cpu_llc_shared_mask(cpu))
> -		cpumask_clear_cpu(cpu, cpu_llc_shared_mask(sibling));
> -	cpumask_clear(cpu_llc_shared_mask(cpu));
> -	cpumask_clear(topology_sibling_cpumask(cpu));
> -	cpumask_clear(topology_core_cpumask(cpu));


It was just pointed out to me that the these masks tend to be used in
the kernel as "sibling & online mask", and "core & online_mask".  The other
values are indeed static and are safe to use.

I'll "self-nak" this and come up with a slightly better patchset.

P.