Re: [PATCH] sched/fair: Prevent cfs_rq from being unthrottled with zero runtime_remaining

[Aaron Lu <ziqianlu@bytedance.com>]

Hi Prateek,

Thanks for taking a look and the suggestion.

On Mon, Sep 29, 2025 at 03:04:03PM +0530, K Prateek Nayak wrote:
> Hello Aaron,
> 
> On 9/29/2025 1:16 PM, Aaron Lu wrote:
> > When a cfs_rq is to be throttled, its limbo list should be empty and
> > that's why there is a warn in tg_throttle_down() for non empty
> > cfs_rq->throttled_limbo_list.
> > 
> > When running a test with the following hierarchy:
> > 
> >           root
> >         /      \
> >         A*     ...
> >      /  |  \   ...
> >         B
> >        /  \
> >       C*
> > 
> > where both A and C have quota settings, that warn on non empty limbo list
> > is triggered for a cfs_rq of C, let's call it cfs_rq_c(and ignore the cpu
> > part of the cfs_rq for the sake of simpler representation).
> > 
> > Debugging showed it happened like this:
> > Task group C is created and quota is set, so in tg_set_cfs_bandwidth(),
> > cfs_rq_c is initialized with runtime_enabled set, runtime_remaining
> > equals to 0 and *unthrottled*. Before any tasks are enqueued to cfs_rq_c,
> > *multiple* throttled tasks can migrate to cfs_rq_c (e.g., due to task
> > group changes). When enqueue_task_fair(cfs_rq_c, throttled_task) is
> > called and cfs_rq_c is in a throttled hierarchy (e.g., A is throttled),
> > these throttled tasks are placed into cfs_rq_c's limbo list by
> > enqueue_throttled_task().
> > 
> > Later, when A is unthrottled, tg_unthrottle_up(cfs_rq_c) enqueues these
> > tasks. The first enqueue triggers check_enqueue_throttle(), and with zero
> > runtime_remaining, cfs_rq_c can be throttled in throttle_cfs_rq() if it
> > can't get more runtime and enters tg_throttle_down(), where the warning
> > is hit due to remaining tasks in the limbo list.
> > 
> > Fix this by calling throttle_cfs_rq() in tg_set_cfs_bandwidth()
> > immediately after enabling bandwidth and setting runtime_remaining = 0.
> > This ensures cfs_rq_c is throttled upfront and cannot enter the enqueue
> > path in an unthrottled state with no runtime.
> > 
> > Also, update outdated comments in tg_throttle_down() since
> > unthrottle_cfs_rq() is no longer called with zero runtime_remaining.
> > 
> > While at it, remove a redundant assignment to se in tg_throttle_down().
> > 
> > Fixes: e1fad12dcb66("sched/fair: Switch to task based throttle model")
> > Signed-off-by: Aaron Lu <ziqianlu@bytedance.com>
> > ---
> >  kernel/sched/core.c  |  9 ++++++++-
> >  kernel/sched/fair.c  | 16 +++++++---------
> >  kernel/sched/sched.h |  1 +
> >  3 files changed, 16 insertions(+), 10 deletions(-)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 7f1e5cb94c536..421166d431fa7 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -9608,7 +9608,14 @@ static int tg_set_cfs_bandwidth(struct task_group *tg,
> >  		cfs_rq->runtime_enabled = runtime_enabled;
> >  		cfs_rq->runtime_remaining = 0;
> >  
> > -		if (cfs_rq->throttled)
> > +		/*
> > +		 * Throttle cfs_rq now or it can be unthrottled with zero
> > +		 * runtime_remaining and gets throttled on its unthrottle path.
> > +		 */
> > +		if (cfs_rq->runtime_enabled && !cfs_rq->throttled)
> > +			throttle_cfs_rq(cfs_rq);
> 
> So one downside of this is throttle_cfs_rq() here can assign bandwidth
> to an empty cfs_rq and a genuine enqueue later on another CPU might not
> find bandwidth thus delaying its execution.

Agree that assign doesn't make sense here.

> 
> Can we instead do a check_enqueue_throttle() in enqueue_throttled_task()
> if we find cfs_rq->throttled_limbo_list to be empty?
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 18a30ae35441..fd2d4dad9c27 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5872,6 +5872,8 @@ static bool enqueue_throttled_task(struct task_struct *p)
>  	 */
>  	if (throttled_hierarchy(cfs_rq) &&
>  	    !task_current_donor(rq_of(cfs_rq), p)) {
> +		if (list_empty(&cfs_rq->throttled_limbo_list))
> +			check_enqueue_throttle(cfs_rq);
>  		list_add(&p->throttle_node, &cfs_rq->throttled_limbo_list);
>  		return true;
>  	}
> ---
> 

Works for me, will follow your suggestion if no other comments, thanks!