Subject: Re: [PATCH] arm64: topology: Avoid the static_branch_{enable|disable} dance

From: Ionela Voinescu <[email-masked]>
Date: Thu, 10 Dec 2020 11:09:06 +0000

On Thursday 10 Dec 2020 at 15:12:25 (+0530), Viresh Kumar wrote:
> Avoid the static_branch_enable() and static_branch_disable() dance by
> redoing the code in a different way. We will be fully invariant here
> only if amu_fie_cpus is set with all present CPUs, use that instead of
> yet another call to topology_scale_freq_invariant().
> 
> This also avoids running rest of the routine if we enabled the static
> branch, followed by a disable.
> 
> Also make the first call to topology_scale_freq_invariant() just when we
> need it, instead of at the top of the routine. This makes it further
> clear on why we need it, i.e. just around enabling AMUs use here.
> 
> Signed-off-by: Viresh Kumar <[email-masked]>
> ---
>  arch/arm64/kernel/topology.c | 21 +++++++++------------
>  1 file changed, 9 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index 7f7d8de325b6..6dedc6ee91cf 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -221,7 +221,7 @@ static DEFINE_STATIC_KEY_FALSE(amu_fie_key);
>  
>  static int __init init_amu_fie(void)
>  {
> -	bool invariance_status = topology_scale_freq_invariant();
> +	bool invariance_status;
>  	cpumask_var_t valid_cpus;
>  	int ret = 0;
>  	int cpu;
> @@ -255,18 +255,15 @@ static int __init init_amu_fie(void)
>  	    cpumask_equal(valid_cpus, cpu_present_mask))
>  		cpumask_copy(amu_fie_cpus, cpu_present_mask);
>  
> -	if (!cpumask_empty(amu_fie_cpus)) {
> -		pr_info("CPUs[%*pbl]: counters will be used for FIE.",
> -			cpumask_pr_args(amu_fie_cpus));
> -		static_branch_enable(&amu_fie_key);
> -	}

This check verifies if there are *any* CPUs for which AMUs can be used for
FIE (!empty mask) -> enable static key.

> +	/* Disallow partial use of counters for frequency invariance */
> +	if (!cpumask_equal(amu_fie_cpus, cpu_present_mask))
> +		goto free_valid_mask;
>  

The replacement verifies if *all* present CPUs support AMUs for FIE and
only then it enables the static key.

Ionela.

> -	/*
> -	 * If the system is not fully invariant after AMU init, disable
> -	 * partial use of counters for frequency invariance.
> -	 */
> -	if (!topology_scale_freq_invariant())
> -		static_branch_disable(&amu_fie_key);
> +	pr_info("CPUs[%*pbl]: counters will be used for FIE.",
> +		cpumask_pr_args(amu_fie_cpus));
> +
> +	invariance_status = topology_scale_freq_invariant();
> +	static_branch_enable(&amu_fie_key);
>  
>  	/*
>  	 * Task scheduler behavior depends on frequency invariance support,
> -- 
> 2.25.0.rc1.19.g042ed3e048af
> 

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