On 10/06/2017 02:36 PM, Jacob Pan wrote: > On Fri, 6 Oct 2017 13:19:45 -0400 > Jason Baron <[email protected]> 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 <[email protected]> >> Cc: Jacob Pan <[email protected]> >> Cc: Len Brown <[email protected]> >> Cc: Rafael J. Wysocki <[email protected]> >> --- >> 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
