linux-kernel - Re: [PATCH] intel_idle: replace conditionals with static_cpu_has(X86_FEATURE

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Date:   Fri, 6 Oct 2017 11:36:33 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Jason Baron <jbaron@...mai.com>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH] intel_idle: replace conditionals with
 static_cpu_has(X86_FEATURE_ARAT)

On Fri,  6 Oct 2017 13:19:45 -0400
Jason Baron <jbaron@...mai.com> wrote:

> If the 'arat' cpu flag is set, then the conditionals in intel_idle()
> that guard calling tick_broadcast_enter()/exit() will never be true.
> Use static_cpu_has(X86_FEATURE_ARAT) to create a fast path to replace
> the conditional.
> 
> Signed-off-by: Jason Baron <jbaron@...mai.com>
> Cc: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> Cc: Len Brown <lenb@...nel.org>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  drivers/idle/intel_idle.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
> index 5dc7ea4..5db5e31 100644
> --- a/drivers/idle/intel_idle.c
> +++ b/drivers/idle/intel_idle.c
> @@ -913,8 +913,7 @@ static __cpuidle int intel_idle(struct
> cpuidle_device *dev, struct cpuidle_state *state =
> &drv->states[index]; unsigned long eax = flg2MWAIT(state->flags);
>  	unsigned int cstate;
> -
> -	cstate = (((eax) >> MWAIT_SUBSTATE_SIZE) &
> MWAIT_CSTATE_MASK) + 1;
> +	bool uninitialized_var(tick);
>  
>  	/*
>  	 * NB: if CPUIDLE_FLAG_TLB_FLUSHED is set, this idle
> transition @@ -923,12 +922,19 @@ static __cpuidle int
> intel_idle(struct cpuidle_device *dev,
>  	 * useful with this knowledge.
>  	 */
>  
> -	if (!(lapic_timer_reliable_states & (1 << (cstate))))
> -		tick_broadcast_enter();
> +	if (!static_cpu_has(X86_FEATURE_ARAT)) {
> +		cstate = (((eax) >> MWAIT_SUBSTATE_SIZE) &
> +				MWAIT_CSTATE_MASK) + 1;
> +		tick = false;
> +		if (!(lapic_timer_reliable_states & (1 <<
> (cstate)))) {
> +			tick = true;
> +			tick_broadcast_enter();
> +		}
> +	}
>  
>  	mwait_idle_with_hints(eax, ecx);
>  
> -	if (!(lapic_timer_reliable_states & (1 << (cstate))))
> +	if (!static_cpu_has(X86_FEATURE_ARAT) && tick)
>  		tick_broadcast_exit();
>  
>  	return index;

Seems better to have a function pointer set up at init time to select
whether we do tick_broadcast or not (two functions). There is no need to
check CPU feature on every entry.