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

[Dou Liyang]

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 <sne...@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.f...@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();
  }