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SubjectRe: [RFC PATCH 07/13] sched: Add core wide task selection and scheduling.
On Thu, May 21, 2020 at 10:35:56PM -0400, Joel Fernandes wrote:
> Discussed a lot with Vineeth. Below is an improved version of the pick_task()
> similification.
>
> It also handles the following "bug" in the existing code as well that Vineeth
> brought up in OSPM: Suppose 2 siblings of a core: rq 1 and rq 2.
>
> In priority order (high to low), say we have the tasks:
> A - untagged (rq 1)
> B - tagged (rq 2)
> C - untagged (rq 2)
>
> Say, B and C are in the same scheduling class.
>
> When the pick_next_task() loop runs, it looks at rq 1 and max is A, A is
> tenantively selected for rq 1. Then it looks at rq 2 and the class_pick is B.
> But that's not compatible with A. So rq 2 gets forced idle.
>
> In reality, rq 2 could have run C instead of idle. The fix is to add C to the
> tag tree as Peter suggested in OSPM.

I like the idea of adding untagged task to the core tree.

> Updated diff below:
>
> ---8<-----------------------
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 005d7f7323e2d..625377f393ed3 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -182,9 +182,6 @@ static void sched_core_enqueue(struct rq *rq, struct task_struct *p)
>
> rq->core->core_task_seq++;
>
> - if (!p->core_cookie)
> - return;
> -
> node = &rq->core_tree.rb_node;
> parent = *node;
>
> @@ -215,7 +212,7 @@ static void sched_core_dequeue(struct rq *rq, struct task_struct *p)
>
> void sched_core_add(struct rq *rq, struct task_struct *p)
> {
> - if (p->core_cookie && task_on_rq_queued(p))
> + if (task_on_rq_queued(p))
> sched_core_enqueue(rq, p);
> }

It appears there are other call sites of sched_core_enqueue() where
core_cookie is checked: cpu_cgroup_fork() and __sched_write_tag().

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