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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Mon, 30 Nov 2020 11:35:15 +0100
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Subject: Re: [PATCH -tip 14/32] sched: migration changes for core scheduling
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vineeth Pillai <viremana@linux.microsoft.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Ingo Molnar <mingo@kernel.org>,
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        Tim Chen <tim.c.chen@intel.com>
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On Wed, 18 Nov 2020 at 00:20, Joel Fernandes (Google)
<joel@joelfernandes.org> wrote:
>
> From: Aubrey Li <aubrey.li@intel.com>
>
>  - Don't migrate if there is a cookie mismatch
>      Load balance tries to move task from busiest CPU to the
>      destination CPU. When core scheduling is enabled, if the
>      task's cookie does not match with the destination CPU's
>      core cookie, this task will be skipped by this CPU. This
>      mitigates the forced idle time on the destination CPU.
>
>  - Select cookie matched idle CPU
>      In the fast path of task wakeup, select the first cookie matched
>      idle CPU instead of the first idle CPU.
>
>  - Find cookie matched idlest CPU
>      In the slow path of task wakeup, find the idlest CPU whose core
>      cookie matches with task's cookie
>
>  - Don't migrate task if cookie not match
>      For the NUMA load balance, don't migrate task to the CPU whose
>      core cookie does not match with task's cookie
>
> Tested-by: Julien Desfossez <jdesfossez@digitalocean.com>
> Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> Signed-off-by: Vineeth Remanan Pillai <viremana@linux.microsoft.com>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/sched/fair.c  | 64 ++++++++++++++++++++++++++++++++++++++++----
>  kernel/sched/sched.h | 29 ++++++++++++++++++++
>  2 files changed, 88 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index de82f88ba98c..ceb3906c9a8a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1921,6 +1921,15 @@ static void task_numa_find_cpu(struct task_numa_env *env,
>                 if (!cpumask_test_cpu(cpu, env->p->cpus_ptr))
>                         continue;
>
> +#ifdef CONFIG_SCHED_CORE
> +               /*
> +                * Skip this cpu if source task's cookie does not match
> +                * with CPU's core cookie.
> +                */
> +               if (!sched_core_cookie_match(cpu_rq(cpu), env->p))
> +                       continue;
> +#endif
> +
>                 env->dst_cpu = cpu;
>                 if (task_numa_compare(env, taskimp, groupimp, maymove))
>                         break;
> @@ -5867,11 +5876,17 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this
>
>         /* Traverse only the allowed CPUs */
>         for_each_cpu_and(i, sched_group_span(group), p->cpus_ptr) {
> +               struct rq *rq = cpu_rq(i);
> +
> +#ifdef CONFIG_SCHED_CORE
> +               if (!sched_core_cookie_match(rq, p))
> +                       continue;
> +#endif
> +
>                 if (sched_idle_cpu(i))
>                         return i;
>
>                 if (available_idle_cpu(i)) {
> -                       struct rq *rq = cpu_rq(i);
>                         struct cpuidle_state *idle = idle_get_state(rq);
>                         if (idle && idle->exit_latency < min_exit_latency) {
>                                 /*
> @@ -6129,8 +6144,18 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>         for_each_cpu_wrap(cpu, cpus, target) {
>                 if (!--nr)
>                         return -1;
> -               if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
> -                       break;
> +
> +               if (available_idle_cpu(cpu) || sched_idle_cpu(cpu)) {
> +#ifdef CONFIG_SCHED_CORE
> +                       /*
> +                        * If Core Scheduling is enabled, select this cpu
> +                        * only if the process cookie matches core cookie.
> +                        */
> +                       if (sched_core_enabled(cpu_rq(cpu)) &&
> +                           p->core_cookie == cpu_rq(cpu)->core->core_cookie)
> +#endif
> +                               break;
> +               }

This makes code unreadable.
Put this coresched specific stuff in an inline function; You can have
a look at what is done with asym_fits_capacity()

>         }
>
>         time = cpu_clock(this) - time;
> @@ -7530,8 +7555,9 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>          * We do not migrate tasks that are:
>          * 1) throttled_lb_pair, or
>          * 2) cannot be migrated to this CPU due to cpus_ptr, or
> -        * 3) running (obviously), or
> -        * 4) are cache-hot on their current CPU.
> +        * 3) task's cookie does not match with this CPU's core cookie
> +        * 4) running (obviously), or
> +        * 5) are cache-hot on their current CPU.
>          */
>         if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
>                 return 0;
> @@ -7566,6 +7592,15 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>                 return 0;
>         }
>
> +#ifdef CONFIG_SCHED_CORE
> +       /*
> +        * Don't migrate task if the task's cookie does not match
> +        * with the destination CPU's core cookie.
> +        */
> +       if (!sched_core_cookie_match(cpu_rq(env->dst_cpu), p))
> +               return 0;
> +#endif
> +
>         /* Record that we found atleast one task that could run on dst_cpu */
>         env->flags &= ~LBF_ALL_PINNED;
>
> @@ -8792,6 +8827,25 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
>                                         p->cpus_ptr))
>                         continue;
>
> +#ifdef CONFIG_SCHED_CORE
> +               if (sched_core_enabled(cpu_rq(this_cpu))) {
> +                       int i = 0;
> +                       bool cookie_match = false;
> +
> +                       for_each_cpu(i, sched_group_span(group)) {
> +                               struct rq *rq = cpu_rq(i);
> +
> +                               if (sched_core_cookie_match(rq, p)) {
> +                                       cookie_match = true;
> +                                       break;
> +                               }
> +                       }
> +                       /* Skip over this group if no cookie matched */
> +                       if (!cookie_match)
> +                               continue;
> +               }
> +#endif

same here, encapsulate this to keep find_idlest_group readable

> +
>                 local_group = cpumask_test_cpu(this_cpu,
>                                                sched_group_span(group));
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index e72942a9ee11..de553d39aa40 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1135,6 +1135,35 @@ static inline raw_spinlock_t *rq_lockp(struct rq *rq)
>
>  bool cfs_prio_less(struct task_struct *a, struct task_struct *b);
>
> +/*
> + * Helper to check if the CPU's core cookie matches with the task's cookie
> + * when core scheduling is enabled.
> + * A special case is that the task's cookie always matches with CPU's core
> + * cookie if the CPU is in an idle core.
> + */
> +static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p)
> +{
> +       bool idle_core = true;
> +       int cpu;
> +
> +       /* Ignore cookie match if core scheduler is not enabled on the CPU. */
> +       if (!sched_core_enabled(rq))
> +               return true;
> +
> +       for_each_cpu(cpu, cpu_smt_mask(cpu_of(rq))) {
> +               if (!available_idle_cpu(cpu)) {
> +                       idle_core = false;
> +                       break;
> +               }
> +       }
> +
> +       /*
> +        * A CPU in an idle core is always the best choice for tasks with
> +        * cookies.
> +        */
> +       return idle_core || rq->core->core_cookie == p->core_cookie;
> +}
> +
>  extern void queue_core_balance(struct rq *rq);
>
>  #else /* !CONFIG_SCHED_CORE */
> --
> 2.29.2.299.gdc1121823c-goog
>