Re: [patch 6/7] sched: use rq->avg_event to resurrect nohzratelimiting

[Mike Galbraith]

Added CC of the person who reported that the last nohz ratelimit patch
caused increased power consumption.

Norbert: can you please test this patch to make sure it's not going to
cause you the same trouble the last one did?  I see no ill effects with
this patch, but then I didn't with the last either.

	TIA,

	-Mike

On Tue, 2011-11-22 at 15:24 +0100, Mike Galbraith wrote:
> Entering nohz at high frequency eats cycles, ratelimit it
> just as we do idle_balance(), and for the same reason.
> 
> Signed-off-by: Mike Galbraith <efault@xxxxxx>
> 
> ---
>  include/linux/sched.h    |    2 ++
>  kernel/sched/core.c      |    7 +++++++
>  kernel/time/tick-sched.c |    2 +-
>  3 files changed, 10 insertions(+), 1 deletion(-)
> 
> Index: linux-3.0-tip/include/linux/sched.h
> ===================================================================
> --- linux-3.0-tip.orig/include/linux/sched.h
> +++ linux-3.0-tip/include/linux/sched.h
> @@ -1988,8 +1988,10 @@ static inline void idle_task_exit(void)
>  
>  #if defined(CONFIG_NO_HZ) && defined(CONFIG_SMP)
>  extern void wake_up_idle_cpu(int cpu);
> +extern int sched_needs_cpu(int cpu);
>  #else
>  static inline void wake_up_idle_cpu(int cpu) { }
> +static inline void sched_needs_cpu(int cpu) { }
>  #endif
>  
>  extern unsigned int sysctl_sched_latency;
> Index: linux-3.0-tip/kernel/sched/core.c
> ===================================================================
> --- linux-3.0-tip.orig/kernel/sched/core.c
> +++ linux-3.0-tip/kernel/sched/core.c
> @@ -587,6 +587,13 @@ static inline bool got_nohz_idle_kick(vo
>  	return idle_cpu(smp_processor_id()) && this_rq()->nohz_balance_kick;
>  }
>  
> +#ifdef CONFIG_SMP
> +int sched_needs_cpu(int cpu)
> +{
> +	return cpu_rq(cpu)->avg_event < sysctl_sched_migration_cost;
> +}
> +#endif
> +
>  #else /* CONFIG_NO_HZ */
>  
>  static inline bool got_nohz_idle_kick(void)
> Index: linux-3.0-tip/kernel/time/tick-sched.c
> ===================================================================
> --- linux-3.0-tip.orig/kernel/time/tick-sched.c
> +++ linux-3.0-tip/kernel/time/tick-sched.c
> @@ -352,7 +352,7 @@ void tick_nohz_stop_sched_tick(int inidl
>  	} while (read_seqretry(&xtime_lock, seq));
>  
>  	if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu) ||
> -	    arch_needs_cpu(cpu)) {
> +	    arch_needs_cpu(cpu) || sched_needs_cpu(cpu)) {
>  		next_jiffies = last_jiffies + 1;
>  		delta_jiffies = 1;
>  	} else {
> 
> 


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