Re: [PATCH v3 07/11] x86/mm: Stop calling leave_mm() in idle code
From: Andy Lutomirski
Date: Wed Jun 21 2017 - 12:21:15 EST
On Wed, Jun 21, 2017 at 2:22 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Tue, 20 Jun 2017, Andy Lutomirski wrote:
>> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
>> index 216d7ec88c0c..2ae43f59091d 100644
>> --- a/drivers/idle/intel_idle.c
>> +++ b/drivers/idle/intel_idle.c
>> @@ -912,16 +912,15 @@ 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;
>> - int cpu = smp_processor_id();
>> cstate = (((eax) >> MWAIT_SUBSTATE_SIZE) & MWAIT_CSTATE_MASK) + 1;
>> - * leave_mm() to avoid costly and often unnecessary wakeups
>> - * for flushing the user TLB's associated with the active mm.
>> + * NB: if CPUIDLE_FLAG_TLB_FLUSHED is set, this idle transition
>> + * will probably flush the TLB. It's not guaranteed to flush
>> + * the TLB, though, so it's not clear that we can do anything
>> + * useful with this knowledge.
> So my understanding here is:
> The C-state transition might flush the TLB, when cstate->flags has
> CPUIDLE_FLAG_TLB_FLUSHED set. The idle transition already took the
> CPU out of the set of CPUs which are remotely flushed, so the
> knowledge about this potential flush is not useful for the kernels
> view of the TLB state.
Indeed. I assume the theory behind the old code was that leave_mm()
was expensive and that CPUIDLE_FLAG_TLB_FLUSHED would be a decent
heuristic for when to do it.