Re: [PATCH] sched/fair: Fix the runtime check in preempt_sync()

[Peter Zijlstra]

On Fri, Nov 06, 2026 at 04:50:00PM +0800, Peng Wang wrote:
> With NEXT_BUDDY enabled, preempt_sync() only lets a WF_SYNC wakeup
> force a reschedule once the current task has run long enough.  It
> computes the runtime as rq_clock_task() - se->exec_start, but
> wakeup_preempt_fair() has already called update_curr_fair() at that
> point, which sets se->exec_start to rq_clock_task().  The result is
> always 0.
> 
> Use sum_exec_runtime - prev_sum_exec_runtime instead, which is the time
> the task has run since it was picked, and drop the now unused rq
> argument.
> 
> Fixes: e837456fdca8 ("sched/fair: Reimplement NEXT_BUDDY to align with EEVDF goals")
> Signed-off-by: Peng Wang <peng_wang@xxxxxxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cfc9822..6164cb7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10090,8 +10090,8 @@ static inline bool set_short_buddy(struct cfs_rq *cfs_rq, struct sched_entity *p
>   * multiple tasks must be woken up.
>   */
>  static inline enum preempt_wakeup_action
> -preempt_sync(struct rq *rq, int wake_flags,
> -	     struct sched_entity *pse, struct sched_entity *se)
> +preempt_sync(int wake_flags, struct sched_entity *pse,
> +	     struct sched_entity *se)
>  {
>  	u64 threshold, delta;
>  
> @@ -10102,7 +10102,7 @@ preempt_sync(struct rq *rq, int wake_flags,
>  	WARN_ON_ONCE(!(wake_flags & WF_TTWU));
>  
>  	threshold = sysctl_sched_migration_cost;
> -	delta = rq_clock_task(rq) - se->exec_start;
> +	delta = se->sum_exec_runtime - se->prev_sum_exec_runtime;
>  	if ((s64)delta < 0)
>  		delta = 0;
>  
> @@ -10226,7 +10226,7 @@ static void wakeup_preempt_fair(struct rq *rq, struct task_struct *p, int wake_f
>  		 * waker and less likely to be cache hot.
>  		 */
>  		if (wake_flags & WF_SYNC)
> -			preempt_action = preempt_sync(rq, wake_flags, pse, se);
> +			preempt_action = preempt_sync(wake_flags, pse, se);
>  	}
>  
>  	switch (preempt_action) {

Well yes. Would you also happen to have some number on how this fix
affects performance of WF_SYNC workloads?