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:
--
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