Re: [PATCH V2] tick-broadcast: Register for hrtimer based broadcast as the fallback broadcast mode

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

On 12/08/2014 04:18 PM, Mark Rutland wrote:
> Hi Preeti,
> 
> On Mon, Dec 08, 2014 at 06:55:43AM +0000, Preeti U Murthy wrote:
>> Commit 5d1638acb9f6 ('tick: Introduce hrtimer based broadcast') added a
>> hrtimer based broadcast mode for those platforms in which local timers stop
>> when CPUs enter deep idle states. The commit expected the platforms to
>> register for this mode explicitly when they lacked a better external device
>> to wake up CPUs in deep idle. Given that more platforms are beginning to use
>> this mode, we can avoid the call to set it up on every platform that requires
>> it, by registering for the hrtimer based broadcast mode in the core code if
>> no better broadcast device is available.
>>
>> This commit also helps detect cases where the platform fails to register for
>> a broadcast device but invokes the help of one when entering deep idle states.
>> Currently we do not handle this situation at all and call the broadcast clock
>> device without checking for its existence. This patch will handle such buggy
>> cases properly.
>>
>> Signed-off-by: Preeti U Murthy <preeti@linux.vnet.ibm.com>
> 
> I've just given this a go on an arm64 platform (Juno) without any
> system-wide clock_event_devices registered, and everything works well
> with CPUs entering and exiting idle states where the cpu-local timers
> lose state. So:
> 
> Tested-by: Mark Rutland <mark.rutland@arm.com>

Thanks!

> 
> One minor thing I noticed when testing was that
> /sys/devices/system/clockevents/broadcast/name contained "(null)",
> because we never set the name field on the clock_event_device. It's
> always been that way, but now might be a good time to change that to
> something like "broadcast_hrtimer".

You mean /sys/devices/system/clockevents/broadcast/current_device right?

> 
> [...]
> 
>> diff --git a/include/linux/clockchips.h b/include/linux/clockchips.h
>> index 2e4cb67..91754b0 100644
>> --- a/include/linux/clockchips.h
>> +++ b/include/linux/clockchips.h
>> @@ -187,11 +187,11 @@ extern int tick_receive_broadcast(void);
>>  #endif
>>  
>>  #if defined(CONFIG_GENERIC_CLOCKEVENTS_BROADCAST) && defined(CONFIG_TICK_ONESHOT)
>> -extern void tick_setup_hrtimer_broadcast(void);
>> +extern int __init tick_setup_hrtimer_broadcast(void);
>>  extern int tick_check_broadcast_expired(void);
>>  #else
>>  static inline int tick_check_broadcast_expired(void) { return 0; }
>> -static inline void tick_setup_hrtimer_broadcast(void) {};
>> +static inline int __init tick_setup_hrtimer_broadcast(void) { return 0; }
>>  #endif
>>  
>>  #ifdef CONFIG_GENERIC_CLOCKEVENTS
>> @@ -207,7 +207,7 @@ static inline void clockevents_resume(void) {}
>>  
>>  static inline int clockevents_notify(unsigned long reason, void *arg) { return 0; }
>>  static inline int tick_check_broadcast_expired(void) { return 0; }
>> -static inline void tick_setup_hrtimer_broadcast(void) {};
>> +static inline int __init tick_setup_hrtimer_broadcast(void) { return 0; }
> 
> With the initcall moved to the driver we have no external users of
> tick_setup_hrtimer_broadcast, so I think we can remove the prototype
> entirely from clockchips.h...
> 
>>  #endif
>>  
>> diff --git a/kernel/time/tick-broadcast-hrtimer.c b/kernel/time/tick-broadcast-hrtimer.c
>> index eb682d5..5c35995 100644
>> --- a/kernel/time/tick-broadcast-hrtimer.c
>> +++ b/kernel/time/tick-broadcast-hrtimer.c
>> @@ -98,9 +98,11 @@ static enum hrtimer_restart bc_handler(struct hrtimer *t)
>>  	return HRTIMER_RESTART;
>>  }
>>  
>> -void tick_setup_hrtimer_broadcast(void)
>> +int __init tick_setup_hrtimer_broadcast(void)
> 
> ...and make it static here.

Yep will do. Sorry I overlooked this.

> 
>>  {
>>  	hrtimer_init(&bctimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
>>  	bctimer.function = bc_handler;
>>  	clockevents_register_device(&ce_broadcast_hrtimer);
>> +	return 0;
>>  }
>> +early_initcall(tick_setup_hrtimer_broadcast);
> 
> Otherwise this looks good to me, thanks for putting this together!

Thanks a lot for the review! Will send out the patch with the above
corrections.

Regards
Preeti U Murthy
> 
> Mark.
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