Re: [PATCH v07 6/9] pmt/numa: Disable arch_update_cpu_topology during CPU readd

[Nathan Fontenot <nfont@linux.vnet.ibm.com>]

On 07/13/2018 03:18 PM, Michael Bringmann wrote:
> pmt/numa: Disable arch_update_cpu_topology during post migration
> CPU readd updates when evaluating device-tree changes after LPM
> to avoid thread deadlocks trying to update node assignments.
> System timing between all of the threads and timers restarted in
> a migrated system overlapped frequently allowing tasks to start
> acquiring resources (get_online_cpus) needed by rebuild_sched_domains.
> Defer the operation of that function until after the CPU readd has
> completed.
> 
> Signed-off-by: Michael Bringmann <mwb@linux.vnet.ibm.com>
> ---
>   arch/powerpc/platforms/pseries/hotplug-cpu.c |    9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> index 1906ee57..df1791b 100644
> --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
> +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> @@ -26,6 +26,7 @@
>   #include <linux/sched.h>	/* for idle_task_exit */
>   #include <linux/sched/hotplug.h>
>   #include <linux/cpu.h>
> +#include <linux/cpuset.h>
>   #include <linux/of.h>
>   #include <linux/slab.h>
>   #include <asm/prom.h>
> @@ -684,9 +685,15 @@ static int dlpar_cpu_readd_by_index(u32 drc_index)
> 
>   	pr_info("Attempting to re-add CPU, drc index %x\n", drc_index);
> 
> +	arch_update_cpu_topology_suspend();
>   	rc = dlpar_cpu_remove_by_index(drc_index, false);
> -	if (!rc)
> +	arch_update_cpu_topology_resume();
> +
> +	if (!rc) {
> +		arch_update_cpu_topology_suspend();
>   		rc = dlpar_cpu_add(drc_index, false);
> +		arch_update_cpu_topology_resume();
> +	}
> 

A couple of questions...Why not disable across the entire remove and add
operations instead of disabling for each operation?

Also, what about other CPU add/remove routines, do they need to do
similar disabling?

-Nathan

>   	if (rc)
>   		pr_info("Failed to update cpu at drc_index %lx\n",
>