Re: sched: Improve load balancing in the presence of idle CPUs

[Tim Chen <tim.c.chen@linux.intel.com>]

On Wed, 2015-04-01 at 22:59 -0700, Jason Low wrote:
> On Wed, 2015-04-01 at 18:04 +0100, Morten Rasmussen wrote:
> > On Wed, Apr 01, 2015 at 07:49:56AM +0100, Preeti U Murthy wrote:
> 
> > > I am sorry I don't quite get this. Can you please elaborate?
> > 
> > I think the scenario is that we are in nohz_idle_balance() and decide to
> > bail out because we have pulled some tasks, but before leaving
> > nohz_idle_balance() we want to check if more balancing is necessary
> > using nohz_kick_needed() and potentially kick somebody to continue.
> 
> Also, below is an example patch.
> 
> (Without the conversion to idle_cpu(), the check for rq->idle_balance
> would not be accurate anymore)
> 
> ---
>  kernel/sched/fair.c |   17 ++++++++++-------
>  1 files changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fdae26e..7749a14 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7620,6 +7620,8 @@ out:
>  }
>  
>  #ifdef CONFIG_NO_HZ_COMMON
> +static inline bool nohz_kick_needed(struct rq *rq);
> +
>  /*
>   * In CONFIG_NO_HZ_COMMON case, the idle balance kickee will do the
>   * rebalancing for all the cpus for whom scheduler ticks are stopped.
> @@ -7629,6 +7631,7 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>  	int this_cpu = this_rq->cpu;
>  	struct rq *rq;
>  	int balance_cpu;
> +	bool done_balancing = false;
>  
>  	if (idle != CPU_IDLE ||
>  	    !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
> @@ -7644,7 +7647,7 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>  		 * balancing owner will pick it up.
>  		 */
>  		if (need_resched())
> -			break;
> +			goto end;
>  
>  		rq = cpu_rq(balance_cpu);
>  
> @@ -7663,9 +7666,12 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>  		if (time_after(this_rq->next_balance, rq->next_balance))
>  			this_rq->next_balance = rq->next_balance;
>  	}
> +	done_balancing = true;
>  	nohz.next_balance = this_rq->next_balance;
>  end:
>  	clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
> +	if (!done_balancing && nohz_kick_needed(this_rq))
> +		nohz_balancer_kick();
>  }
>  
>  

I think we can get rid of the done_balancing boolean 
and make it a bit easier to read if we change the above code to

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bcfe320..08317dc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7557,8 +7557,13 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
                 * work being done for other cpus. Next load
                 * balancing owner will pick it up.
                 */
-               if (need_resched())
-                       break;
+               if (need_resched()) {
+                       /* preparing to bail, kicking other cpu to continue */
+                       clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
+                       if (nohz_kick_needed(this_rq))
+                               nohz_balance_kick();
+                       return;
+               }
 
                rq = cpu_rq(balance_cpu);

Thanks.

Tim