linux-kernel - Re: [PATCH 02/10] tick/nohz: Add tick_nohz_full_this

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Date:   Tue, 16 Mar 2021 14:05:14 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Ti Zhou <x2019cwm@...x.ca>, Yunfeng Ye <yeyunfeng@...wei.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 02/10] tick/nohz: Add tick_nohz_full_this_cpu()

On Tue, Mar 16, 2021 at 01:28:01PM +0100, Peter Zijlstra wrote:
> On Thu, Mar 11, 2021 at 01:37:00PM +0100, Frederic Weisbecker wrote:
> > Optimize further the check for local full dynticks CPU. Testing directly
> > tick_nohz_full_cpu(smp_processor_id()) is suboptimal because the
> > compiler first fetches the CPU number and only then processes the
> > static key.
> > 
> > It's best to evaluate the static branch before anything.
> 
> Or you do tricky things like this ;-)

Good point!

I'll check the asm diff to see if that really does what we want.
I expect it will.

Thanks.

> 
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index 7340613c7eff..bd4a6b055b80 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -185,13 +185,12 @@ static inline bool tick_nohz_full_enabled(void)
>  	return tick_nohz_full_running;
>  }
>  
> -static inline bool tick_nohz_full_cpu(int cpu)
> -{
> -	if (!tick_nohz_full_enabled())
> -		return false;
> -
> -	return cpumask_test_cpu(cpu, tick_nohz_full_mask);
> -}
> +#define tick_nohz_full_cpu(_cpu) ({					\
> +	bool __ret = false;						\
> +	if (tick_nohz_full_enabled())					\
> +		__ret = cpumask_test_cpu((_cpu), tick_nohz_full_mask);	\
> +	__ret;								\
> +})
>  
>  static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask)
>  {
> 
>