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SubjectRe: [PATCH] sched: update last_balance only when actually balanced
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.


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