Subject: Re: [PATCH V2] cpuidle: Handle tick_broadcast_enter() failure gracefully

From: Preeti U Murthy <[email-masked]>
Date: Fri, 08 May 2015 13:06:32 +0530

On 05/08/2015 02:20 AM, Rafael J. Wysocki wrote:
> On Thursday, May 07, 2015 11:17:21 PM Preeti U Murthy wrote:
>> When a CPU has to enter an idle state where tick stops, it makes a call
>> to tick_broadcast_enter(). The call will fail if this CPU is the
>> broadcast CPU. Today, under such a circumstance, the arch cpuidle code
>> handles this CPU.  This is not convincing because not only are we not
>> aware what the arch cpuidle code does, but we also do not account for
>> the idle state residency time and usage of such a CPU.
>>
>> This scenario can be handled better by simply asking the cpuidle
>> governor to choose an idle state where in ticks do not stop. To
>> accommodate this change move the setting of runqueue idle state from the
>> core to the cpuidle driver, else the rq->idle_state will be set wrong.
>>
>> Signed-off-by: Preeti U Murthy <[email-masked]>
>> ---
>> Changes from V1: https://lkml.org/lkml/2015/5/7/24
>> Rebased on the latest linux-pm/bleeding-edge
>>
>>  drivers/cpuidle/cpuidle.c          |   21 +++++++++++++++++----
>>  drivers/cpuidle/governors/ladder.c |   13 ++++++++++---
>>  drivers/cpuidle/governors/menu.c   |    6 +++++-
>>  include/linux/cpuidle.h            |    6 +++---
>>  include/linux/sched.h              |   16 ++++++++++++++++
>>  kernel/sched/core.c                |   17 +++++++++++++++++
>>  kernel/sched/fair.c                |    2 +-
>>  kernel/sched/idle.c                |    8 +-------
>>  kernel/sched/sched.h               |   24 ------------------------
>>  9 files changed, 70 insertions(+), 43 deletions(-)
>>
>> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
>> index 8c24f95..b7e86f4 100644
>> --- a/drivers/cpuidle/cpuidle.c
>> +++ b/drivers/cpuidle/cpuidle.c
>> @@ -21,6 +21,7 @@
>>  #include <linux/module.h>
>>  #include <linux/suspend.h>
>>  #include <linux/tick.h>
>> +#include <linux/sched.h>
>>  #include <trace/events/power.h>
>>  
>>  #include "cpuidle.h"
>> @@ -168,10 +169,17 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
>>  	 * CPU as a broadcast timer, this call may fail if it is not available.
>>  	 */
>>  	if (broadcast && tick_broadcast_enter()) {
>> -		default_idle_call();
>> -		return -EBUSY;
>> +		index = cpuidle_select(drv, dev, !broadcast);
> 
> No, you can't do that.
> 
> This code path may be used by suspend-to-idle and that should not call
> cpuidle_select().
> 
> What's needed here seems to be a fallback mechanism like "choose the
> deepest state shallower than X and such that it won't stop the tick".
> You don't really need to run a full governor for that.

Agreed. Makes the patch a lot simpler as well. I have sent out V3 doing
this.

Thank you

Regards
Preeti U Murthy