Re: [PATCH 03/18] sched: Make NOHZ CFS bandwidth checks follow proxy donor

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Aug 31, 2026 at 03:42:13PM +0200, Andrea Righi wrote:
> Proxy execution separates the scheduling context in rq->donor from the
> physical execution context in rq->curr. sched_can_stop_tick() checks the
> latter for CFS bandwidth constraints and only does so when nr_running is
> one.
> 
> A retained proxy donor keeps both the donor and mutex owner queued. The
> check therefore misses a constrained FAIR donor and may stop the tick
> while its runtime still needs to be enforced.
> 
> Check the selected donor instead and remove the nr_running restriction.
> The donor being a queued FAIR task is sufficient to require bandwidth
> accounting regardless of other runnable tasks.
> 
> Fixes: af0c8b2bf67b ("sched: Split scheduler and execution contexts")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Link: https://lore.kernel.org/r/20260713164807.E5ED21F00A3A@smtp.kernel.org
> Acked-by: John Stultz <jstultz@google.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  kernel/sched/core.c | 35 ++++++++++++++++++-----------------
>  kernel/sched/fair.c | 12 +++++++-----
>  2 files changed, 25 insertions(+), 22 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 237d216382f46..14d0d5c884393 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1419,11 +1419,8 @@ static void nohz_csd_func(void *info)
>  #endif /* CONFIG_NO_HZ_COMMON */
>  
>  #ifdef CONFIG_NO_HZ_FULL
> -static inline bool __need_bw_check(struct rq *rq, struct task_struct *p)
> +static inline bool __need_bw_check(struct task_struct *p)
>  {
> -	if (rq->nr_running != 1)
> -		return false;
> -
>  	if (p->sched_class != &fair_sched_class)
>  		return false;
>  
> @@ -1441,6 +1438,14 @@ bool sched_can_stop_tick(struct rq *rq)
>  	if (rq->dl.dl_nr_running)
>  		return false;
>  
> +	/*
> +	 * The selected scheduling context can be a constrained FAIR donor even
> +	 * when rq->curr is an RT task. Check it before the RT fast paths below,
> +	 * which may report that the tick can stop for a throttled RT context.
> +	 */

I am most confused... how can we ever have rq->donor be FAIR and
rq->curr be RT? That makes no sense. If an RT task is runnable, pick
should just straight up pick that.

> +	if (__need_bw_check(rq->donor) && cfs_task_bw_constrained(rq->donor))
> +		return false;
> +


> @@ -7114,7 +7107,7 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
>   */
>  static void __sched notrace __schedule(int sched_mode)
>  {
> -	struct task_struct *prev, *next;
> +	struct task_struct *prev, *next, *tick_donor;
>  	/*
>  	 * On PREEMPT_RT kernel, SM_RTLOCK_WAIT is noted
>  	 * as a preemption by schedule_debug() and RCU.
> @@ -7168,6 +7161,7 @@ static void __sched notrace __schedule(int sched_mode)
>  	rq->clock_update_flags <<= 1;
>  	update_rq_clock(rq);
>  	rq->clock_update_flags = RQCF_UPDATED;
> +	tick_donor = rq->donor;
>  
>  	switch_count = &prev->nivcsw;
>  
> @@ -7243,6 +7237,13 @@ static void __sched notrace __schedule(int sched_mode)
>  	clear_tsk_need_resched(prev);
>  	clear_preempt_need_resched();
>  keep_resched:
> +	/*
> +	 * Enqueue and dequeue updates can evaluate the outgoing donor. Refresh
> +	 * the dependency after selecting a different scheduling context.
> +	 */
> +	if (rq->donor != tick_donor)
> +		sched_update_tick_dependency(rq);

Please keep all the proxy specific bits inside the one
sched_proxy_exec() branch above.