Re: [PATCH] sched: update last_balance only when actually balanced

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 22, 2014 at 02:21:21AM -0400, Rik van Riel wrote:
> Currently rebalance_domains will happily update sd->last_balance
> even if should_we_balance() decides that this CPU should not do
> any load balancing for the sd.
> 
> This can prevent the CPU that should load balance for the sd from
> doing load balancing, which can delay active balancing essentially
> forever.
> 
> The solution is to only update sd->last_balance when load_balance()
> actually did something.
> 
> Signed-off-by: Rik van Riel <riel@redhat.com>
> ---
>  kernel/sched/fair.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 45943b2..4f7fee2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7187,7 +7187,9 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle)
>  				 */
>  				idle = idle_cpu(cpu) ? CPU_IDLE : CPU_NOT_IDLE;
>  			}
> -			sd->last_balance = jiffies;
> +			/* Only update if we actually balanced. */
> +			if (continue_balancing)
> +				sd->last_balance = jiffies;
>  			interval = get_sd_balance_interval(sd, idle != CPU_IDLE);
>  		}
>  		if (need_serialize)

Did you actually see any difference with this patch?

The reason I'm asking is that the sched_domains are per cpu, so the
above assignment is only visible to that particular cpu, not other cpus.