On Tue, Dec 03, 2013 at 09:05:56AM +0000, Alex Shi wrote:
> Task migration happens when target just a bit less then source cpu load.
> To reduce such situation happens, aggravate the target cpu load with
> sd->imbalance_pct/100.
>
> This patch removes the hackbench thread regression on Daniel's
> Intel Core2 server.
>
> a5d6e63 +patch1~3 +patch1~4
> hackbench -T -s 4096 -l 1000 -g 10 -f 40
> 27.914" 38.694" 28.587"
> 28.390" 38.341" 29.513"
> 28.048" 38.626" 28.706"
>
> Signed-off-by: Alex Shi <alex....@linaro.org>
> ---
> kernel/sched/fair.c | 18 ++++++++++++------
> 1 file changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bccdd89..c49b7ba 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -978,7 +978,7 @@ static inline unsigned long group_weight(struct
> task_struct *p, int nid)
>
> static unsigned long weighted_cpuload(const int cpu);
> static unsigned long source_load(int cpu);
> -static unsigned long target_load(int cpu);
> +static unsigned long target_load(int cpu, int imbalance_pct);
> static unsigned long power_of(int cpu);
> static long effective_load(struct task_group *tg, int cpu, long wl, long wg);
>
> @@ -3809,11 +3809,17 @@ static unsigned long source_load(int cpu)
> * Return a high guess at the load of a migration-target cpu weighted
> * according to the scheduling class and "nice" value.
> */
> -static unsigned long target_load(int cpu)
> +static unsigned long target_load(int cpu, int imbalance_pct)
> {
> struct rq *rq = cpu_rq(cpu);
> unsigned long total = weighted_cpuload(cpu);
>
> + /*
> + * without cpu_load decay, in most of time cpu_load is same as total
> + * so we need to make target a bit heavier to reduce task migration
> + */
> + total = total * imbalance_pct / 100;
> +
> if (!sched_feat(LB_BIAS))
> return total;
>
> @@ -4033,7 +4039,7 @@ static int wake_affine(struct sched_domain *sd, struct
> task_struct *p, int sync)
> this_cpu = smp_processor_id();
> prev_cpu = task_cpu(p);
> load = source_load(prev_cpu);
> - this_load = target_load(this_cpu);
> + this_load = target_load(this_cpu, 100);
>
> /*
> * If sync wakeup then subtract the (maximum possible)
> @@ -4089,7 +4095,7 @@ static int wake_affine(struct sched_domain *sd, struct
> task_struct *p, int sync)
>
> if (balanced ||
> (this_load <= load &&
> - this_load + target_load(prev_cpu) <= tl_per_task)) {
> + this_load + target_load(prev_cpu, 100) <= tl_per_task)) {
> /*
> * This domain has SD_WAKE_AFFINE and
> * p is cache cold in this domain, and
> @@ -4135,7 +4141,7 @@ find_idlest_group(struct sched_domain *sd, struct
> task_struct *p, int this_cpu)
> if (local_group)
> load = source_load(i);
> else
> - load = target_load(i);
> + load = target_load(i, sd->imbalance_pct);
Don't you apply imbalance_pct twice here? Later on in
find_idlest_group() you have:
if (!idlest || 100*this_load < imbalance*min_load)
return NULL;
where min_load comes from target_load().
>
> avg_load += load;
> }
> @@ -5478,7 +5484,7 @@ static inline void update_sg_lb_stats(struct lb_env
> *env,
>
> /* Bias balancing toward cpus of our domain */
> if (local_group)
> - load = target_load(i);
> + load = target_load(i, env->sd->imbalance_pct);
You probably have the same problem here.
Morten
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