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

Hi Paul:
Does this reason make sense to you? Or you still prefer to add
ACCESS_ONCE() in the new tick_nohz_check()?

>
>>
>> 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
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