Re: high power consumption in recent kernels

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2010-09-30 at 16:27 +0800, Alex,Shi wrote:
> Thanks Norbert! 
> Mike&Peter:
> would you like to add signed-off for the following patch?

I would really rather see the nohz decision move into the whole cpuidle
governor thing, which can make a much better cost vs benefit decision.

Thomas, Arjan?

> ---
>     sched: nohz_ratelimit function refresh
>     
>     The nohz_ratelimit() function that written by Mike Galbraith
>     can bring about more than 10% throughput for netperf TCP/UDP RR
>     when scheduling cross-cpu. It did this by reducing down to nohz
>     mode chance.
>     But the patch also reduce CPU chance to nohz mode after
>     interrupt processed, that cause Norbert's system have 4 watts power
>     increase(the system have about 100 int/sec and with a light load).
>     That is not acceptable for a laptop.
>     
>     So, I remove the nohz_ratelimit from irq_exit(). and then the
>     Norbert's system back to low power consumption.
>     
>     Tested-by: Norbert Preining <preining@logic.at>
>     Signed-off-by: Alex Shi <alex.shi@intel.com>
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 1e2a6db..a4dbb37 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -274,8 +274,13 @@ extern cpumask_var_t nohz_cpu_mask;
>  #if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ)
>  extern void select_nohz_load_balancer(int stop_tick);
>  extern int get_nohz_timer_target(void);
> +extern int nohz_ratelimit(int cpu);
>  #else
>  static inline void select_nohz_load_balancer(int stop_tick) { }
> +static inline int nohz_ratelimit(int cpu)
> +{
> +	return 0;
> +}
>  #endif
>  
>  /*
> diff --git a/kernel/sched.c b/kernel/sched.c
> index dc85ceb..132a21c 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -1182,6 +1182,16 @@ static void resched_task(struct task_struct *p)
>  		smp_send_reschedule(cpu);
>  }
>  
> +int nohz_ratelimit(int cpu)
> +{
> +	struct rq *rq = cpu_rq(cpu);
> +	u64 diff = rq->clock - rq->nohz_stamp;
> +
> +	rq->nohz_stamp = rq->clock;
> +
> +	return diff < (NSEC_PER_SEC / HZ) >> 1;
> +}
> +
>  static void resched_cpu(int cpu)
>  {
>  	struct rq *rq = cpu_rq(cpu);
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 3e216e0..19a7914 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -325,7 +325,7 @@ void tick_nohz_stop_sched_tick(int inidle)
>  	} while (read_seqretry(&xtime_lock, seq));
>  
>  	if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu) ||
> -	    arch_needs_cpu(cpu)) {
> +		arch_needs_cpu(cpu) || (inidle && nohz_ratelimit(cpu))) {
>  		next_jiffies = last_jiffies + 1;
>  		delta_jiffies = 1;
>  	} else {
> 
>