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

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Date:   Fri, 6 Oct 2017 14:09:13 -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 14:41:07 -0400
Jason Baron <jbaron@...mai.com> wrote:

> On 10/06/2017 02:36 PM, Jacob Pan wrote:
> > 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.
> >   
> 
> Hi,
> 
> static_cpu_has() uses alternatives patching, so the cpu feature is not
> tested on every entry. With the arat flag set you just have two nops
> in the straight-line code path with this patch.
> 
Thanks for explaining, i didn't know it was self modifying.

Acked-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>