Re: [PATCH] smpboot: correctly update number of booted cores

[Dou Liyang <douly.fnst@cn.fujitsu.com>]

At 02/22/2018 04:50 AM, Samuel Neves wrote:
> Without this fix, /proc/cpuinfo will display an incorrect amount
> of CPU cores, after bringing them offline and online again, as
> exemplified below:
> 
> $ cat /proc/cpuinfo | grep cores
> cpu cores	: 4
> cpu cores	: 8
> cpu cores	: 8
> cpu cores	: 20
> cpu cores	: 4
> cpu cores	: 3
> cpu cores	: 2
> cpu cores	: 2
> 
> This patch fixes this by always zeroing the booted_cores variable
> upon turning off a logical CPU.
> 
> Signed-off-by: Samuel Neves <sneves@dei.uc.pt>

Yes, tested it in Qemu with 15 hot-pluggable CPUs, like below
   ...
   -smp 1,maxcpus=16,sockets=2,cores=4,threads=2
   ...

When a new CPU bringups a new core, for each core in package, Linux 
increments the booted_cores for this new cpu in set_cpu_sibling_map().
Due to the uncleared booted_cores, this incorrect number of CPU cores
will be shown.

Tested-by: Dou Liyang <douly.fnst@cn.fujitsu.com>

> ---
>   arch/x86/kernel/smpboot.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 9eee25d07586..ff99e2b6fc54 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1437,6 +1437,7 @@ static void remove_siblinginfo(int cpu)
>   	cpumask_clear(topology_sibling_cpumask(cpu));
>   	cpumask_clear(topology_core_cpumask(cpu));
>   	c->cpu_core_id = 0;
> +	c->booted_cores = 0;
>   	cpumask_clear_cpu(cpu, cpu_sibling_setup_mask);
>   	recompute_smt_state();
>   }
>