Re: [PATCH 3/6] sched_ext: Implement SCX_ENQ_IMMED

[Andrea Righi <arighi@nvidia.com>]

Hi Tejun,

On Fri, Mar 13, 2026 at 01:31:11AM -1000, Tejun Heo wrote:
> Add SCX_ENQ_IMMED enqueue flag for local DSQ insertions. Once a task is
> dispatched with IMMED, it either gets on the CPU immediately and stays on it,
> or gets reenqueued back to the BPF scheduler. It will never linger on a local
> DSQ behind other tasks or on a CPU taken by a higher-priority class.
> 
> rq_is_open() uses rq->next_class to determine whether the rq is available,
> and wakeup_preempt_scx() triggers reenqueue when a higher-priority class task
> arrives. These capture all higher class preemptions. Combined with reenqueue
> points in the dispatch path, all cases where an IMMED task would not execute
> immediately are covered.
> 
> SCX_TASK_IMMED persists in p->scx.flags until the next fresh enqueue, so the
> guarantee survives SAVE/RESTORE cycles. If preempted while running,
> put_prev_task_scx() reenqueues through ops.enqueue() with
> SCX_TASK_REENQ_PREEMPTED instead of silently placing the task back on the
> local DSQ.
> 
> This enables tighter scheduling latency control by preventing tasks from
> piling up on local DSQs. It also enables opportunistic CPU sharing across
> sub-schedulers - without this, a sub-scheduler can stuff the local DSQ of a
> shared CPU, making it difficult for others to use.
> 
> v2: - Rewrite is_curr_done() as rq_is_open() using rq->next_class and
>       implement wakeup_preempt_scx() to achieve complete coverage of all
>       cases where IMMED tasks could get stranded.
>     - Track IMMED persistently in p->scx.flags and reenqueue
>       preempted-while-running tasks through ops.enqueue().
>     - Bound deferred reenq cycles (SCX_REENQ_LOCAL_MAX_REPEAT).
>     - Misc renames, documentation.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
...
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 2f59265b9b57..85cdb65a5a66 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -406,6 +406,62 @@ static bool bypass_dsp_enabled(struct scx_sched *sch)
>  	return unlikely(atomic_read(&sch->bypass_dsp_enable_depth));
>  }
>  
> +/**
> + * rq_is_open - Is the rq available for immediate execution of an SCX task?
> + * @rq: rq to test
> + * @enq_flags: optional %SCX_ENQ_* of the task being enqueued
> + *
> + * Returns %true if @rq is currently open for executing an SCX task. After a
> + * %false return, @rq is guaranateed to invoke SCX dispatch path at least once

nit: s/guaranateed/guaranteed/

Thanks,
-Andrea