linux-kernel - Re: [PATCH] arm64: topology: Avoid the static_branch

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Message-ID: <20201210123527.ccl4k56hl5mc3jag@vireshk-i7>
Date:   Thu, 10 Dec 2020 18:05:27 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Ionela Voinescu <ionela.voinescu@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: topology: Avoid the
 static_branch_{enable|disable} dance

On 10-12-20, 11:09, Ionela Voinescu wrote:
> 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 <viresh.kumar@...aro.org>
> > ---
> >  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.
> 
> > -	/*
> > -	 * If the system is not fully invariant after AMU init, disable
> > -	 * partial use of counters for frequency invariance.
> > -	 */
> > -	if (!topology_scale_freq_invariant())

I mis-read something here, as shared in the other thread, so yeah I
need to think again about this patch.

> > -		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
> > 

-- 
viresh