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SubjectRe: [PATCH -tip 14/32] sched: migration changes for core scheduling
On Tue, Nov 17, 2020 at 06:19:44PM -0500, Joel Fernandes (Google) 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
> +

Any reason this is under an #ifdef? In sched_core_cookie_match() won't
the check for sched_core_enabled() do the right thing even when
CONFIG_SCHED_CORE is not enabed?

> 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;
> + }
> }
>
> 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
> +
> 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)) {

I was looking at this snippet and comparing this to is_core_idle(), the
major difference is the check for vcpu_is_preempted(). Do we want to
call the core as non idle if any vcpu was preempted on this 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;
> +}
> +

Balbir Singh.

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