Re: [RFC PATCH 1/5] sched/fair: Add push task framework
From: K Prateek Nayak
Date: Thu Apr 10 2025 - 11:28:59 EST
Hello Peter,
Thank you for the review.
On 4/10/2025 3:30 PM, Peter Zijlstra wrote:
On Wed, Apr 09, 2025 at 11:15:35AM +0000, K Prateek Nayak wrote:
+static void fair_add_pushable_task(struct rq *rq, struct task_struct *p)
+{
+ if (fair_push_task(p)) {
+ plist_del(&p->pushable_tasks, &rq->cfs.pushable_tasks);
+ plist_node_init(&p->pushable_tasks, p->prio);
I gotta aks, why do we care about ordering the push list on p->prio for
fair?
This bit was carried over from Vincent's series. I assume it was
inspired by the RT, DL's push mechanism. Perhaps fair can get away with
MRU ordering similar to rq->cfs_tasks?
+ plist_add(&p->pushable_tasks, &rq->cfs.pushable_tasks);
+ }
+}
+
/*
* select_task_rq_fair: Select target runqueue for the waking task in domains
* that have the relevant SD flag set. In practice, this is SD_BALANCE_WAKE,
@@ -8914,6 +8978,12 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
put_prev_entity(cfs_rq, pse);
set_next_entity(cfs_rq, se);
+ /*
+ * The previous task might be eligible for being pushed on
+ * another cpu if it is still active.
+ */
+ fair_add_pushable_task(rq, prev);
+
__set_next_task_fair(rq, p, true);
So __set_next_task_fair() removes the current task from the pushable list
when it is picked ...
}
@@ -8986,6 +9056,13 @@ static void put_prev_task_fair(struct rq *rq, struct task_struct *prev, struct t
cfs_rq = cfs_rq_of(se);
put_prev_entity(cfs_rq, se);
}
+
+ /*
+ * The previous task might be eligible for being pushed on another cpu
+ * if it is still active.
+ */
+ fair_add_pushable_task(rq, prev);
+
}
These two are tricky; while they will work with a push balance callback,
they will cause some pain with pulling from the push lists; a-la
newidle.
Notably, we must be sure to check ->on_cpu.
... so we should be safe here. Other considerations are covered by
detach_task() but pushable task lists to only contain queued tasks
that can run on more than one CPU and Vincent was cautious enough to
place a bunch of WARN_ON_ONCE() in pick_next_pushable_fair_task() to
catch those cases :)
Perhaps later patches add this, let me continue reading...
--
Thanks and Regards,
Prateek