Re: [PATCH -next] arch_topology: Fix cache attributes detection in the CPU hotplug path

[Ionela Voinescu]

Hi Sudeep,

Thank you for the fix!

On Wednesday 13 Jul 2022 at 14:33:44 (+0100), Sudeep Holla wrote:
> init_cpu_topology() is called only once at the boot and all the cache
> attributes are detected early for all the possible CPUs. However when
> the CPUs are hotplugged out, the cacheinfo gets removed. While the
> attributes are added back when the CPUs are hotplugged back in as part
> of CPU hotplug state machine, it ends up called quite late after the
> update_siblings_masks() are called in the secondary_start_kernel()
> resulting in wrong llc_sibling_masks.
> 
> Move the call to detect_cache_attributes() inside update_siblings_masks()
> to ensure the cacheinfo is updated before the LLC sibling masks are
> updated. This will fix the incorrect LLC sibling masks generated when
> the CPUs are hotplugged out and hotplugged back in again.
> 
> Reported-by: Ionela Voinescu <ionela.voinescu@xxxxxxx>
> Signed-off-by: Sudeep Holla <sudeep.holla@xxxxxxx>
> ---
>  drivers/base/arch_topology.c | 16 ++++++----------
>  1 file changed, 6 insertions(+), 10 deletions(-)
> 
> Hi Conor,
> 
> Ionela reported an issue with the CPU hotplug and as a fix I need to
> move the call to detect_cache_attributes() which I had thought to keep
> it there from first but for no reason had moved it to init_cpu_topology().
> 
> Wonder if this fixes the -ENOMEM on RISC-V as this one is called on the
> cpu in the secondary CPUs init path while init_cpu_topology executed
> detect_cache_attributes() for all possible CPUs much earlier. I think
> this might help as the percpu memory might be initialised in this case.
> 
> Anyways give this a try, also test the CPU hotplug and check if nothing
> is broken on RISC-V. We noticed this bug only on one platform while
> 
> Regards,
> Sudeep
> 
> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index 441e14ac33a4..0424b59b695e 100644
> --- a/drivers/base/arch_topology.c
> +++ b/drivers/base/arch_topology.c
> @@ -732,7 +732,11 @@ const struct cpumask *cpu_clustergroup_mask(int cpu)
>  void update_siblings_masks(unsigned int cpuid)
>  {
>  	struct cpu_topology *cpu_topo, *cpuid_topo = &cpu_topology[cpuid];
> -	int cpu;
> +	int cpu, ret;
> +
> +	ret = detect_cache_attributes(cpuid);
> +	if (ret)
> +		pr_info("Early cacheinfo failed, ret = %d\n", ret);
>  	/* update core and thread sibling masks */
>  	for_each_online_cpu(cpu) {
> @@ -821,7 +825,7 @@ __weak int __init parse_acpi_topology(void)
>  #if defined(CONFIG_ARM64) || defined(CONFIG_RISCV)
>  void __init init_cpu_topology(void)
>  {
> -	int ret, cpu;
> +	int ret;
>  	reset_cpu_topology();
>  	ret = parse_acpi_topology();
> @@ -836,13 +840,5 @@ void __init init_cpu_topology(void)
>  		reset_cpu_topology();
>  		return;
>  	}
> -
> -	for_each_possible_cpu(cpu) {
> -		ret = detect_cache_attributes(cpu);
> -		if (ret) {
> -			pr_info("Early cacheinfo failed, ret = %d\n", ret);
> -			break;
> -		}
> -	}
>  }
>  #endif
> --2.37.1
> 

Reviewed-by: Ionela Voinescu <ionela.voinescu@xxxxxxx>
Tested-by:  Ionela Voinescu <ionela.voinescu@xxxxxxx>

Kind regards,
Ionela.