linux-kernel - Re: [PATCH] sched: Prefer sibiling only if local group is under-utilized

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Date:   Thu, 23 Mar 2017 09:37:56 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Prefer sibiling only if local group is under-utilized

On 22 March 2017 at 18:57, Srikar Dronamraju <srikar@...ux.vnet.ibm.com> wrote:
> If the child domain prefers tasks to go siblings, the local group could
> end up pulling tasks to itself even if the local group is almost equally
> loaded as the source group.
>
> Lets assume a 4 core,smt==2 machine running 5 thread ebizzy workload.
> Everytime, local group has capacity and source group has atleast 2 threads,
> local group tries to pull the task. This causes the threads to constantly
> move between different cores. This is even more profound if the cores have
> more threads, like in Power 8, smt 8 mode.
>
> Fix this by only allowing local group to pull a task, if the source group
> has more number of tasks than the local group.
>
> Signed-off-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>

Acked-by: Vincent Guittot <vincent.guittot@...aro.org>

> ---
> Here are the relevant perf stat numbers of a 22 core,smt 8 Power 8 machine.
>
> Without patch:
>  Performance counter stats for 'ebizzy -t 22 -S 100' (5 runs):
>
>              1,440      context-switches          #    0.001 K/sec                    ( +-  1.26% )
>                366      cpu-migrations            #    0.000 K/sec                    ( +-  5.58% )
>              3,933      page-faults               #    0.002 K/sec                    ( +- 11.08% )
>
>  Performance counter stats for 'ebizzy -t 48 -S 100' (5 runs):
>
>              6,287      context-switches          #    0.001 K/sec                    ( +-  3.65% )
>              3,776      cpu-migrations            #    0.001 K/sec                    ( +-  4.84% )
>              5,702      page-faults               #    0.001 K/sec                    ( +-  9.36% )
>
>  Performance counter stats for 'ebizzy -t 96 -S 100' (5 runs):
>
>              8,776      context-switches          #    0.001 K/sec                    ( +-  0.73% )
>              2,790      cpu-migrations            #    0.000 K/sec                    ( +-  0.98% )
>             10,540      page-faults               #    0.001 K/sec                    ( +-  3.12% )
>
> With patch:
>  Performance counter stats for 'ebizzy -t 22 -S 100' (5 runs):
>
>              1,133      context-switches          #    0.001 K/sec                    ( +-  4.72% )
>                123      cpu-migrations            #    0.000 K/sec                    ( +-  3.42% )
>              3,858      page-faults               #    0.002 K/sec                    ( +-  8.52% )
>
>  Performance counter stats for 'ebizzy -t 48 -S 100' (5 runs):
>
>              2,169      context-switches          #    0.000 K/sec                    ( +-  6.19% )
>                189      cpu-migrations            #    0.000 K/sec                    ( +- 12.75% )
>              5,917      page-faults               #    0.001 K/sec                    ( +-  8.09% )
>
>  Performance counter stats for 'ebizzy -t 96 -S 100' (5 runs):
>
>              5,333      context-switches          #    0.001 K/sec                    ( +-  5.91% )
>                506      cpu-migrations            #    0.000 K/sec                    ( +-  3.35% )
>             10,792      page-faults               #    0.001 K/sec                    ( +-  7.75% )
> ---
>  kernel/sched/fair.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6559d19..a6dd010 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7496,6 +7496,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  {
>         struct sched_domain *child = env->sd->child;
>         struct sched_group *sg = env->sd->groups;
> +       struct sg_lb_stats *local = &sds->local_stat;
>         struct sg_lb_stats tmp_sgs;
>         int load_idx, prefer_sibling = 0;
>         bool overload = false;
> @@ -7512,7 +7513,7 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>                 local_group = cpumask_test_cpu(env->dst_cpu, sched_group_cpus(sg));
>                 if (local_group) {
>                         sds->local = sg;
> -                       sgs = &sds->local_stat;
> +                       sgs = local;
>
>                         if (env->idle != CPU_NEWLY_IDLE ||
>                             time_after_eq(jiffies, sg->sgc->next_update))
> @@ -7536,8 +7537,8 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>                  * the tasks on the system).
>                  */
>                 if (prefer_sibling && sds->local &&
> -                   group_has_capacity(env, &sds->local_stat) &&
> -                   (sgs->sum_nr_running > 1)) {
> +                   group_has_capacity(env, local) &&
> +                   (sgs->sum_nr_running > local->sum_nr_running + 1)) {
>                         sgs->group_no_capacity = 1;
>                         sgs->group_type = group_classify(sg, sgs);
>                 }

> --
> 1.8.3.1
>