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SubjectRe: [PATCH] intel_idle: replace conditionals with static_cpu_has(X86_FEATURE_ARAT)
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On 10/06/2017 02:36 PM, Jacob Pan wrote:
> On Fri, 6 Oct 2017 13:19:45 -0400
> Jason Baron <jbaron@akamai.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@akamai.com>
>> Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
>> Cc: Len Brown <lenb@kernel.org>
>> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.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,

-Jason

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