Re: [PATCH V2 2/2] arm64: topology: Reorder init_amu_fie() a bit

From: Ionela Voinescu <ionela.voinescu@arm•com>
To: Viresh Kumar <viresh.kumar@linaro•org>
Cc: Catalin Marinas <catalin.marinas@arm•com>,
	Will Deacon <will@kernel•org>,
	linux-kernel@vger•kernel.org,
	linux-arm-kernel@lists•infradead.org,
	Vincent Guittot <vincent.guittot@linaro•org>
Subject: Re: [PATCH V2 2/2] arm64: topology: Reorder init_amu_fie() a bit
Date: Mon, 14 Dec 2020 14:00:38 +0000	[thread overview]
Message-ID: <20201214140038.GB15405@arm.com> (raw)
In-Reply-To: <202f775d57bd143602f9100ba3d9619d15d43409.1607617625.git.viresh.kumar@linaro.org>

Hey,

On Thursday 10 Dec 2020 at 21:59:23 (+0530), Viresh Kumar wrote:
> This patch does a couple of optimizations in init_amu_fie(), like early
> exits from paths where we don't need to continue any further, moving the
> calls to topology_scale_freq_invariant() just when we need
> them, instead of at the top of the routine, and avoiding calling it for
> the third time.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro•org>
> ---
> V2:
> - The enable/disable dance is actually required, just made a bunch of
>   other optimizations to make it look better.
> 
>  arch/arm64/kernel/topology.c | 22 ++++++++++++++--------
>  1 file changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index ebadc73449f9..1ebdb667f0d1 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 prev, now;

Nit: can you move this below valid_cpus? It makes the section nicer if
they are in decreasing order of line length.

>  	cpumask_var_t valid_cpus;
>  	int ret = 0;
>  	int cpu;
> @@ -249,18 +249,24 @@ static int __init init_amu_fie(void)
>  	if (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);
> -	}
> +	if (cpumask_empty(amu_fie_cpus))
> +		goto free_valid_mask;
> +
> +	prev = topology_scale_freq_invariant();
> +	static_branch_enable(&amu_fie_key);

I think there could be a potential problem here (it would be unlikely
but why not fix it :) ). It was in the code before your changes.

When we enable amu_fie_key here, topology_scale_freq_tick() could be
called for AMU CPUs, which will compute and set a scale factor. Later
on, if we happen to find the system not invariant, we disable counter
based invariance, but a scale factor might have been set already for a
CPU, which would and should have returned 1024 otherwise (the
initialisation value of freq_scale).

Therefore, while here, you could instead do the following:

cpufreq_inv = cpufreq_supports_freq_invariance();

if (!cpufreq_inv &&
    !cpumask_subset(cpu_online_mask, amu_fie_cpus))
    goto free_valid_mask;

static_branch_enable(&amu_fie_key);

pr_info(..);

if (!cpufreq_inv)
    rebuild_sched_domains_energy();

What do you think?

I can submit this separately, if you don't want the hassle.

Thanks,
Ionela.

> +	now = topology_scale_freq_invariant();
>  
>  	/*
>  	 * If the system is not fully invariant after AMU init, disable
>  	 * partial use of counters for frequency invariance.
>  	 */
> -	if (!topology_scale_freq_invariant())
> +	if (!now) {
>  		static_branch_disable(&amu_fie_key);
> +		goto free_valid_mask;
> +	}
> +
> +	pr_info("CPUs[%*pbl]: counters will be used for FIE.",
> +		cpumask_pr_args(amu_fie_cpus));
>  
>  	/*
>  	 * Task scheduler behavior depends on frequency invariance support,
> @@ -268,7 +274,7 @@ static int __init init_amu_fie(void)
>  	 * a result of counter initialisation and use, retrigger the build of
>  	 * scheduling domains to ensure the information is propagated properly.
>  	 */
> -	if (invariance_status != topology_scale_freq_invariant())
> +	if (prev != now)
>  		rebuild_sched_domains_energy();
>  
>  free_valid_mask:
> -- 
> 2.25.0.rc1.19.g042ed3e048af
> 

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