Re: [RFC] optimize the locking in the rebalance_domains()

[Vlad Zolotarov <vlad@scalemp.com>]

On Sun, 2012-07-22 at 19:23 +0300, Vlad Zolotarov wrote:
> Ingo, we've noticed that rebalance_domains() will try to take a lock
> every time it's called (every jiffy) 

This is of course when NOHZ is off... ;)

> if SD_SERIALIZE is set (which is a
> default configuration). This is done regardless the fact that maybe
> there hasn't passed enough time since the last rebalancing in which case
> there is no need to take a lock the first place.
> 
> The above creates a heavy false sharing problem on the "balancing"
> spin-lock on large SMP systems: try_lock() is implemented with an
> (atomic) xchng instruction which invalidates the cache line "balancing"
> belongs to and therefore creates an intensive cross-NUMA-nodes traffic.
> 
> The below patch will minimize the above phenomena to the time slots it's
> really needed, namely when the "interval" has really passed.
> 
> Pls., comment.
> thanks,
> vlad
> 
> ---
>  kernel/sched/fair.c |   20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c099cc6..6777d38 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4689,6 +4689,9 @@ static void rebalance_domains(int cpu, enum cpu_idle_type idle)
>  		interval = msecs_to_jiffies(interval);
>  		interval = clamp(interval, 1UL, max_load_balance_interval);
>  
> +                if (!time_after_eq(jiffies, sd->last_balance + interval))
> +			goto out;
> +
>  		need_serialize = sd->flags & SD_SERIALIZE;
>  
>  		if (need_serialize) {
> @@ -4696,16 +4699,15 @@ static void rebalance_domains(int cpu, enum cpu_idle_type idle)
>  				goto out;
>  		}
>  
> -		if (time_after_eq(jiffies, sd->last_balance + interval)) {
> -			if (load_balance(cpu, rq, sd, idle, &balance)) {
> -				/*
> -				 * We've pulled tasks over so either we're no
> -				 * longer idle.
> -				 */
> -				idle = CPU_NOT_IDLE;
> -			}
> -			sd->last_balance = jiffies;
> +		if (load_balance(cpu, rq, sd, idle, &balance)) {
> +			/*
> +			 * We've pulled tasks over so either we're no
> +			 * longer idle.
> +			 */
> +			idle = CPU_NOT_IDLE;
>  		}
> +		sd->last_balance = jiffies;
> +
>  		if (need_serialize)
>  			spin_unlock(&balancing);
>  out: