Enabling tickless kernel feature in ARM

[Haojian Zhuang]

On Wed, Apr 27, 2011 at 11:04 AM, Vikram Narayanan <vikram186 at gmail.com> wrote:
> Dear Haojian,
>
> On Tue, Apr 26, 2011 at 10:04 PM, Haojian Zhuang
> <haojian.zhuang at gmail.com> wrote:
>>
>> On Tue, Apr 26, 2011 at 9:55 PM, Vikram Narayanan <vikram186 at gmail.com> wrote:
>> > Hi,
>> >
>> > I want to enable the dynamic tick aka tickless kernel feature for the
>> > BSP which I am developing. What are all the architecture dependent
>> > changes that needs to be done for implementing it.
>> > There was one struct by the name dyn_tick_timer, which is located
>> > under linux/include/asm-arm/mach/time.h file. (which is removed from v
>> > 2.6.27)
>> > Now my question is that, are all the dyn_tick_timer functionalities
>> > are moved to a generic interface somewhere?
>> CONFIG_NO_HZ is used instead.
>
> I am aware of the CONFIG_NO_HZ option. I am asking about the
> implementation of it. To put in a more clear way.
> There was a structure in the kernel source
> linux/include/asm-arm/mach/time.h before 2.6.27
>
> struct dyn_tick_timer {
>          spinlock_t      lock;
>          unsigned int    state;                  /* Current state */
>          int             (*enable)(void);        /* Enables dynamic tick */
>          int             (*disable)(void);       /* Disables dynamic tick */
>          void            (*reprogram)(unsigned long); /* Reprograms
> the timer */
>          int             (*handler)(int, void *);
>  };
>
> To enable dynamic tick timer, one should implement functions for
> enable, disable and reprogram. But this structure is no more used in
> the new kernel source. In 2007 linux symposium, 3 authors talked about
> implementing the tickless feature using HPET timers. Now my question
> where does the implementation lies in the ARM architecture.
>
irq_exit(). While irq handler is finished, timer interrupt should also
occur. And
jiffies may not sync to latest ticks. The process flow is in below.
asm_do_IRQ() --> irq_exit() --> tick_nohz_stop_sched_tick() -->
tick_do_update_jiffies64()

In tick_do_update_jiffies64(), jiffies will be updated whatever the
idle time is 1 jiffies or more.

>>
>> > Are the clock source and clockevent device timers enough to take care
>> > of the tickless feature?
>> There's no relationship between NOHZ and clocksource/clockevent.
>> >
>> > -
>> > Vikram
>> >
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>> >
>