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Subject[Question] Linux CFS sched_entity

I am studying Linux CFS recently. After numerous searches, I am still
unable to get rid of the following question. Finally I decide to bother
this group for the answer.

In group scheduling, sched_entity can represent both "task group" and "task",
indicated by "se->my_q".
When CFS tries to pick a new task, it first selects a "task group" and then
selects a "task" from that "task group":
static struct task_struct *pick_next_task_fair(struct rq *rq)
struct task_struct *p;
struct cfs_rq *cfs_rq = &rq->cfs;
struct sched_entity *se;

... ...
do {
se = pick_next_entity(cfs_rq);
set_next_entity(cfs_rq, se);
cfs_rq = group_cfs_rq(se);
} while (cfs_rq);

p = task_of(se);
... ...
Therefore, cfs_rq->curr should *always* represent a "task" here, right?

However, cfs_rq->curr seems *not* to be a "task" sometimes:
static void update_curr(struct cfs_rq *cfs_rq)
struct sched_entity *curr = cfs_rq->curr;

... ...
if (entity_is_task(curr)) { /* WHY? */
struct task_struct *curtask = task_of(curr);

trace_sched_stat_runtime(curtask, delta_exec, curr->vruntime);
cpuacct_charge(curtask, delta_exec);
account_group_exec_runtime(curtask, delta_exec);
... ...

My question is:
In what situations does cfs_rq->curr point to a task_group?
What's the sense? I thought it should always contain a "real" task.

Appreciated for anyone who is able to help. Thanks in advance!

Luwei Cheng
CS Student
The University of Hong Kong

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