linux-kernel - Re: [PATCH 1/2] Tick: Introduce tick_nohz

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Message-ID: <52783EFA.5080901@intel.com>
Date:	Tue, 05 Nov 2013 08:42:34 +0800
From:	Lan Tianyu <tianyu.lan@...el.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	rjw@...ysocki.net, viresh.kumar@...aro.org, dipankar@...ibm.com,
	fweisbec@...il.com, youquan.song@...el.com, riel@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] Tick: Introduce tick_nohz_check() to check nohz enable
 status

On 2013年11月04日 17:52, Paul E. McKenney wrote:
> On Mon, Nov 04, 2013 at 11:13:12AM +0800, Lan Tianyu wrote:
>> On 2013年10月29日 18:29, Lan Tianyu wrote:
>>> 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@...el.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.
>>
>> Hi Paul:
>> 	Does this reason make sense to you? Or you still prefer to add
>> ACCESS_ONCE() in the new tick_nohz_check()?
> 
> I prefer the ACCESS_ONCE().  It adds little or no overhead, and Frederic
> has been heard to say that he mkight allow tick_nohz_check() to change
> in the future.
> 

Ok. I will update it.

> 							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
>>>>>
>>>>
>>>
>>
>>
>> -- 
>> Best regards
>> Tianyu Lan
>>
> 


-- 
Best regards
Tianyu Lan
--
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