Re: [PATCH 1/2] Tick: Introduce tick_nohz_check() to check nohz enable status

[Lan Tianyu <tianyu.lan@intel.com>]

On 10/29/2013 05:51 PM, Paul E. McKenney wrote:
> On Tue, Oct 29, 2013 at 04:48:56PM +0800, Lan Tianyu wrote:
>> In some cases, nohz enable status needs to be checked. E.G, in RCU and cpufreq
>> ondemand governor. So add tick_nohz_check() to return tick_nohz_enabled value
>> And use tick_nohz_check() instead of referencing tick_nohz_enabled in the rcutree_plugin.h.
>>
>> Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
>
> NACK on the rcutree change unless you put the ACCESS_ONCE() in.
>
> Or is there some reason that ACCESS_ONCE() is not needed?  If so, what
> is that reason?

Hi Paul:

Thanks for review. When I change this code, I find the tick_nohz_enabled
isn't changed dynamically. It's only changed during parsing kernel
params when "nohz=off/on" is set. Except this, it will not be changed.
So I ignored ACCESS_ONCE(). If necessary, I can add it back.

>
> 							Thanx, Paul
>
>> ---
>>   include/linux/tick.h     | 2 ++
>>   kernel/rcutree_plugin.h  | 4 +---
>>   kernel/time/tick-sched.c | 8 +++++++-
>>   3 files changed, 10 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/tick.h b/include/linux/tick.h
>> index 5128d33..a9c5374 100644
>> --- a/include/linux/tick.h
>> +++ b/include/linux/tick.h
>> @@ -136,6 +136,7 @@ static inline int tick_nohz_tick_stopped(void)
>>   extern void tick_nohz_idle_enter(void);
>>   extern void tick_nohz_idle_exit(void);
>>   extern void tick_nohz_irq_exit(void);
>> +extern int tick_nohz_check(void);
>>   extern ktime_t tick_nohz_get_sleep_length(void);
>>   extern u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time);
>>   extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
>> @@ -155,6 +156,7 @@ static inline ktime_t tick_nohz_get_sleep_length(void)
>>
>>   	return len;
>>   }
>> +static inline int tick_nohz_check(void) { return 0; }
>>   static inline u64 get_cpu_idle_time_us(int cpu, u64 *unused) { return -1; }
>>   static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
>>   # endif /* !CONFIG_NO_HZ_COMMON */
>> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
>> index 130c97b..af167ec 100644
>> --- a/kernel/rcutree_plugin.h
>> +++ b/kernel/rcutree_plugin.h
>> @@ -1627,8 +1627,6 @@ module_param(rcu_idle_gp_delay, int, 0644);
>>   static int rcu_idle_lazy_gp_delay = RCU_IDLE_LAZY_GP_DELAY;
>>   module_param(rcu_idle_lazy_gp_delay, int, 0644);
>>
>> -extern int tick_nohz_enabled;
>> -
>>   /*
>>    * Try to advance callbacks for all flavors of RCU on the current CPU.
>>    * Afterwards, if there are any callbacks ready for immediate invocation,
>> @@ -1718,7 +1716,7 @@ static void rcu_prepare_for_idle(int cpu)
>>   	int tne;
>>
>>   	/* Handle nohz enablement switches conservatively. */
>> -	tne = ACCESS_ONCE(tick_nohz_enabled);
>> +	tne = tick_nohz_check();
>>   	if (tne != rdtp->tick_nohz_enabled_snap) {
>>   		if (rcu_cpu_has_callbacks(cpu, NULL))
>>   			invoke_rcu_core(); /* force nohz to see update. */
>> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
>> index 3612fc7..d381a22 100644
>> --- a/kernel/time/tick-sched.c
>> +++ b/kernel/time/tick-sched.c
>> @@ -361,7 +361,13 @@ void __init tick_nohz_init(void)
>>   /*
>>    * NO HZ enabled ?
>>    */
>> -int tick_nohz_enabled __read_mostly  = 1;
>> +static int tick_nohz_enabled __read_mostly  = 1;
>> +
>> +int tick_nohz_check(void)
>> +{
>> +	return	tick_nohz_enabled;
>> +}
>> +EXPORT_SYMBOL_GPL(tick_nohz_check);
>>
>>   /*
>>    * Enable / Disable tickless mode
>> --
>> 1.8.4.rc0.1.g8f6a3e5.dirty
>>
>