Subject: Re: [PATCH] perf: Fix topology_sibling_cpumask check warning on ARM

From: Steven Price <[email-masked]>
Date: Thu, 12 Sep 2024 16:53:15 +0100

On 12/09/2024 15:50, [email-masked] wrote:
> From: Kan Liang <[email-masked]>
> 
> The below warning is triggered when building with arm
> multi_v7_defconfig.
> 
> kernel/events/core.c: In function 'perf_event_setup_cpumask':
> kernel/events/core.c:14012:13: warning: the comparison will always
> evaluate as 'true' for the address of 'thread_sibling' will never be
> NULL [-Waddress]
> 14012 |         if (!topology_sibling_cpumask(cpu)) {
> 
> The perf_event_init_cpu() may be invoked at the early boot stage, while
> the topology_*_cpumask hasn't been initialized yet. The check is to
> specially handle the case, and initialize the perf_online_<domain>_masks
> on the boot CPU.
> X86 uses a per-cpu cpumask pointer, which could be NULL at the early
> boot stage. However, ARM uses a global variable, which never be NULL.
> 
> Use perf_online_mask as an indicator instead. Only initialize the
> perf_online_<domain>_masks when perf_online_mask is empty.
> 
> Fix a typo as well.
> 
> Fixes: 4ba4f1afb6a9 ("perf: Generic hotplug support for a PMU with a scope")
> Reported-by: Stephen Rothwell <[email-masked]>
> Closes: https://lore.kernel.org/lkml/[email-masked]/
> Reported-by: Steven Price <[email-masked]>
> Closes: https://lore.kernel.org/lkml/[email-masked]/
> Signed-off-by: Kan Liang <[email-masked]>

Tested-by: Steven Price <[email-masked]>

Thanks,
Steve

> ---
>  kernel/events/core.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 7a028474caef..20e97c1aa4d6 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -13954,21 +13954,19 @@ static void perf_event_setup_cpumask(unsigned int cpu)
>  	struct cpumask *pmu_cpumask;
>  	unsigned int scope;
>  
> -	cpumask_set_cpu(cpu, perf_online_mask);
> -
>  	/*
>  	 * Early boot stage, the cpumask hasn't been set yet.
>  	 * The perf_online_<domain>_masks includes the first CPU of each domain.
> -	 * Always uncondifionally set the boot CPU for the perf_online_<domain>_masks.
> +	 * Always unconditionally set the boot CPU for the perf_online_<domain>_masks.
>  	 */
> -	if (!topology_sibling_cpumask(cpu)) {
> +	if (cpumask_empty(perf_online_mask)) {
>  		for (scope = PERF_PMU_SCOPE_NONE + 1; scope < PERF_PMU_MAX_SCOPE; scope++) {
>  			pmu_cpumask = perf_scope_cpumask(scope);
>  			if (WARN_ON_ONCE(!pmu_cpumask))
>  				continue;
>  			cpumask_set_cpu(cpu, pmu_cpumask);
>  		}
> -		return;
> +		goto end;
>  	}
>  
>  	for (scope = PERF_PMU_SCOPE_NONE + 1; scope < PERF_PMU_MAX_SCOPE; scope++) {
> @@ -13983,6 +13981,8 @@ static void perf_event_setup_cpumask(unsigned int cpu)
>  		    cpumask_any_and(pmu_cpumask, cpumask) >= nr_cpu_ids)
>  			cpumask_set_cpu(cpu, pmu_cpumask);
>  	}
> +end:
> +	cpumask_set_cpu(cpu, perf_online_mask);
>  }
>  
>  int perf_event_init_cpu(unsigned int cpu)