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From: Nathan Lynch <nathanl@linux.ibm.com>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH 1/1] powerpc/smp: Set numa node before updating mask
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References: <20210401154200.150077-1-srikar@linux.vnet.ibm.com>
Date: Thu, 01 Apr 2021 17:51:05 -0500
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Cc: Gautham R Shenoy <ego@linux.vnet.ibm.com>,
 Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
 Peter Zijlstra <peterz@infradead.org>, Scott Cheloha <cheloha@linux.ibm.com>,
 Geetika Moolchandani <Geetika.Moolchandani1@ibm.com>,
 Ingo Molnar <mingo@kernel.org>, Laurent Dufour <ldufour@linux.vnet.ibm.com>,
 linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
 Valentin Schneider <valentin.schneider@arm.com>
Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
Sender: "Linuxppc-dev"
 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Hi Srikar,

Thanks for figuring this out.

Srikar Dronamraju <srikar@linux.vnet.ibm.com> writes:
>
> Some of the per-CPU masks use cpu_cpu_mask as a filter to limit the search
> for related CPUs. On a dlpar add of a CPU, update cpu_cpu_mask before
> updating the per-CPU masks. This will ensure the cpu_cpu_mask is updated
> correctly before its used in setting the masks. Setting the numa_node will
> ensure that when cpu_cpu_mask() gets called, the correct node number is
> used. This code movement helped fix the above call trace.
>
>
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 5a4d59a1070d..1a99d75679a8 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1521,6 +1521,9 @@ void start_secondary(void *unused)
>  
>  	vdso_getcpu_init();
>  #endif
> +	set_numa_node(numa_cpu_lookup_table[cpu]);
> +	set_numa_mem(local_memory_node(numa_cpu_lookup_table[cpu]));
> +
>  	/* Update topology CPU masks */
>  	add_cpu_to_masks(cpu);
>  
> @@ -1539,9 +1542,6 @@ void start_secondary(void *unused)
>  			shared_caches = true;
>  	}
>  
> -	set_numa_node(numa_cpu_lookup_table[cpu]);
> -	set_numa_mem(local_memory_node(numa_cpu_lookup_table[cpu]));
> -

Regardless of your change: at boot time, this set of calls to
set_numa_node() and set_numa_mem() is redundant, right? Because
smp_prepare_cpus() has:

	for_each_possible_cpu(cpu) {
		...
		if (cpu_present(cpu)) {
			set_cpu_numa_node(cpu, numa_cpu_lookup_table[cpu]);
			set_cpu_numa_mem(cpu,
				local_memory_node(numa_cpu_lookup_table[cpu]));
		}

I would rather that, when onlining a CPU that happens to have been
dynamically added after boot, we enter start_secondary() with conditions
equivalent to those at boot time. Or as close to that as is practical.

So I'd suggest that pseries_add_processor() be made to update
these things when the CPUs are marked present, before onlining them.