Re: [PATCH] sched/fair: Untangle NEXT_BUDDY and pick_next_task()

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 29, 2024 at 11:15:41AM +0100, Peter Zijlstra wrote:
> On Fri, Nov 29, 2024 at 10:55:00AM +0100, Peter Zijlstra wrote:
> 
> > Anyway..  I'm sure I started a patch series cleaning up the whole next
> > buddy thing months ago (there's more problems here), but I can't seem to
> > find it in a hurry :/
> 
> There was this..

And this I think.

Adam, what was the reason you were enabling NEXT_BUDDY in the first
place?

I think someone (Ingo?) was proposing we kill the wakeup preempt thing;
and I suspect you don't actually care about that but instead want either
the cgroup or the yield_to_task()/KVM thing working.

---
Subject: sched/fair: Add CGROUP_BUDDY feature
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri Nov 29 10:49:45 CET 2024

Add a feature to toggle the cgroup optimization.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/fair.c     |    3 ++-
 kernel/sched/features.h |    8 +++++++-
 2 files changed, 9 insertions(+), 2 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7126,7 +7126,8 @@ static int dequeue_entities(struct rq *r
 			 * Bias pick_next to pick a task from this cfs_rq, as
 			 * p is sleeping when it is within its sched_slice.
 			 */
-			if (task_sleep && se && !throttled_hierarchy(cfs_rq))
+			if (sched_feat(CGROUP_BUDDY) &&
+			    task_sleep && se && !throttled_hierarchy(cfs_rq))
 				set_next_buddy(se);
 			break;
 		}
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -32,11 +32,17 @@ SCHED_FEAT(PREEMPT_SHORT, true)
 SCHED_FEAT(NEXT_BUDDY, false)
 
 /*
+ * Optimization for cgroup scheduling where a dequeue + pick tries
+ * to share as much of the hierarchy as possible.
+ */
+SCHED_FEAT(CGROUP_BUDDY, true)
+
+/*
  * Allow completely ignoring cfs_rq->next; which can be set from various
  * places:
  *   - NEXT_BUDDY (wakeup preemption)
  *   - yield_to_task()
- *   - cgroup dequeue / pick
+ *   - CGROUP_BUDDY (cgroup dequeue / pick)
  */
 SCHED_FEAT(PICK_BUDDY, true)