Subject: Re: [PATCH] idle/tick-broadcast: Exit cpu idle poll loop when cleared from tick_broadcast_force_mask

From: Preeti U Murthy <[email-masked]>
Date: Tue, 20 Jan 2015 16:55:02 +0530

On 01/20/2015 04:51 PM, Thomas Gleixner wrote:
> On Mon, 19 Jan 2015, Preeti U Murthy wrote:
>> An idle cpu enters cpu_idle_poll() if it is set in the tick_broadcast_force_mask.
>> This is so that it does not incur the overhead of entering idle states when it is expected
>> to be woken up anytime then through a broadcast IPI. The condition that forces an exit out
>> of the idle polling is the check on setting of the TIF_NEED_RESCHED flag for the idle thread.
>>
>> When the broadcast IPI does arrive, it is not guarenteed that the handler sets the
>> TIF_NEED_RESCHED flag. Hence although the cpu is cleared in the tick_broadcast_force_mask,
>> it continues to loop in cpu_idle_poll unnecessarily wasting power. Hence exit the idle
>> poll loop if the tick_broadcast_force_mask gets cleared and enter idle states.
>>
>> Of course if the cpu has entered cpu_idle_poll() on being asked to poll explicitly,
>> it continues to poll till it is asked to reschedule.
>>
>> Signed-off-by: Preeti U Murthy <[email-masked]>
>> ---
>>
>>  kernel/sched/idle.c |    3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
>> index c47fce7..aaf1c1d 100644
>> --- a/kernel/sched/idle.c
>> +++ b/kernel/sched/idle.c
>> @@ -47,7 +47,8 @@ static inline int cpu_idle_poll(void)
>>  	rcu_idle_enter();
>>  	trace_cpu_idle_rcuidle(0, smp_processor_id());
>>  	local_irq_enable();
>> -	while (!tif_need_resched())
>> +	while (!tif_need_resched() &&
>> +		(cpu_idle_force_poll || tick_check_broadcast_expired()))
> 
> You explain the tick_check_broadcast_expired() bit, but what about the
> cpu_idle_force_poll part?

The last few lines which say "Of course if the cpu has entered
cpu_idle_poll() on being asked to poll explicitly, it continues to poll
till it is asked to reschedule" explains the cpu_idle_force_poll part.
Perhaps I should s/poll explicitly/do cpu_idle_force_poll ?

Regards
Preeti U Murthy
> Thanks,
> 	tglx