Re: [PATCH 1/3] dynticks - implement no idle hz for x86

[Srivatsa Vaddagiri <vatsa@in.ibm.com>]

Nish,
	I did some study of how s390 and ARM are architected and have
some comments as a result of that.

On Thu, Sep 08, 2005 at 03:08:54PM -0700, Nishanth Aravamudan wrote:
> struct dyntick_timer {
> 	unsigned int state;
> 	unsigned long max_skip;
> 	unsigned long min_skip;
> 	int (*init) (void);
> 	void (*enable_dyn_tick) (void);
> 	void (*disable_dyn_tick) (void);
> 	unsigned long (*reprogram) (unsigned long); /* return number of ticks skipped */
> 	unsigned long (*recover_time) (int, void *, struct pt_regs *); /* handler in arm */
> 	/* following empty in UP */
> 	void (*enter_all_cpus_idle) (int);
> 	void (*exit_all_cpus_idle) (int);
> 	spinlock_t lock;
> };

The usage of 'lock' probably needs to be made clear. I intended it to be used 
for mainly serializing enter/exit_all_cpus_idle routines.

Considering that not all architectures have such routines, then the use
of spinlock can be entirely within the arch code. Maybe the 'enter' routine
is invoked as part of 'reprogram' routine (when the last CPU goes down)
and 'exit' routine is invoked as part of dyn_tick_interrupt() (when
coming out of all_idle_state), both being serialized using the spinlock?

Another interesting point is that I expected recover_time to be
invoked only as part of 'exit_all_cpus_idle', but s390 seems to 
have unconditional call to account_ticks (for recovering time) on
any CPU that wakes up. I guess it will be a no-op if other CPUs
were active.

We probably also need to document how 'reprogram' will be invoked 
- with xtime_lock held or not. Again s390 does not seem to require it
while ARM is using one. I think we should let the arch code take
xtime_lock if they deem it necessary.

> extern void dyntick_timer_register(struct dyntick_timer *new_dyntick_timer);
>  /* so do we need this?
>  	Maybe it can just be static to dyntick.c and all the callable
> 	functions will call-down to the structure members? */
> extern struct dyntick_timer *current_dyntick_timer;

I don't think this can be static - since the low-level arch-code
will need access to, for example, 'recover_time'/'handler' 
and 'enter/exit_all_cpus_idle' routines?


> extern struct tick_source * __init arch_select_tick_source(void);
> /* calls select_tick_source(), then calls tick_source_register() */
> extern void __init dyn_tick_init(void);

Hmm ..I think just tick_source_register is sufficient ..we can do 
let the arch-code select what tick source it wants and call
register with the selected source ..

>From a point of getting this reviewed by arch-maintainers, I think it will 
help if a new version of this interface is posted and point out how the 
existing s390/ARM interfaces will be affected. I could help out if you are busy.

-- 


Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017