Re: [PATCH v2 sched_ext/for-7.3 27/36] sched_ext: Route task slice writes through set_task_slice()

[Andrea Righi <arighi@nvidia.com>]

Hi Tejun,

On Sun, Jul 05, 2026 at 03:40:49PM -1000, Tejun Heo wrote:
> A later change makes every write to p->scx.slice also drop a pending
> out-of-band slice request, which needs all such writes to go through one
> place. Introduce set_task_slice() and route the existing direct writes
> through it. No functional change - the helper only assigns p->scx.slice.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
>  kernel/sched/ext/ext.c | 32 +++++++++++++++++++-------------
>  1 file changed, 19 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> index 9fa4cfbc75c8..ff997ba881b0 100644
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
> @@ -1153,6 +1153,12 @@ static void touch_core_sched_dispatch(struct rq *rq, struct task_struct *p)
>  #endif
>  }
>  
> +/* set @p's slice, every write to p->scx.slice goes through here */

nit: maybe we should say all "BPF-triggered writes to p->scx.slice", or
something along these lines, just for clarity, update_curr_scx() still
decrements curr->scx.slice directly.

Thanks,
-Andrea

> +static void set_task_slice(struct task_struct *p, u64 slice)
> +{
> +	p->scx.slice = slice;
> +}
> +
>  static void update_curr_scx(struct rq *rq)
>  {
>  	struct task_struct *curr = rq->curr;
> @@ -1238,7 +1244,7 @@ static void dsq_dec_nr(struct scx_dispatch_q *dsq, struct task_struct *p)
>  
>  static void refill_task_slice_dfl(struct scx_sched *sch, struct task_struct *p)
>  {
> -	p->scx.slice = READ_ONCE(sch->slice_dfl);
> +	set_task_slice(p, READ_ONCE(sch->slice_dfl));
>  	__scx_add_event(sch, SCX_EV_REFILL_SLICE_DFL, 1);
>  }
>  
> @@ -1333,7 +1339,7 @@ static void rq_owned_post_enq(struct scx_sched *sch, struct rq *rq,
>  
>  	if ((enq_flags & SCX_ENQ_PREEMPT) && p != rq->curr &&
>  	    rq->curr->sched_class == &ext_sched_class) {
> -		rq->curr->scx.slice = 0;
> +		set_task_slice(rq->curr, 0);
>  		resched_curr(rq);
>  	}
>  }
> @@ -2039,7 +2045,7 @@ static void yield_task_scx(struct rq *rq)
>  	if (SCX_HAS_OP(sch, yield))
>  		SCX_CALL_OP_2TASKS_RET(sch, yield, rq, p, NULL);
>  	else
> -		p->scx.slice = 0;
> +		set_task_slice(p, 0);
>  }
>  
>  static bool yield_to_task_scx(struct rq *rq, struct task_struct *to)
> @@ -3273,7 +3279,7 @@ static void task_tick_scx(struct rq *rq, struct task_struct *curr, int queued)
>  	 * we can't trust the slice management or ops.core_sched_before().
>  	 */
>  	if (scx_bypassing(sch, cpu_of(rq))) {
> -		curr->scx.slice = 0;
> +		set_task_slice(curr, 0);
>  		touch_core_sched(rq, curr);
>  	} else if (SCX_HAS_OP(sch, tick)) {
>  		SCX_CALL_OP_TASK(sch, tick, rq, curr);
> @@ -3412,7 +3418,7 @@ static void scx_disable_task(struct scx_sched *sch, struct task_struct *p)
>  	 * control, after ops.disable() has observed their final values.
>  	 */
>  	p->scx.dsq_vtime = 0;
> -	p->scx.slice = 0;
> +	set_task_slice(p, 0);
>  
>  	/*
>  	 * Verify the task is not in BPF scheduler's custody. If flag
> @@ -7033,7 +7039,7 @@ static void scx_root_enable_workfn(struct kthread_work *work)
>  			queue_flags |= DEQUEUE_CLASS;
>  
>  		scoped_guard (sched_change, p, queue_flags) {
> -			p->scx.slice = READ_ONCE(sch->slice_dfl);
> +			set_task_slice(p, READ_ONCE(sch->slice_dfl));
>  			p->sched_class = new_class;
>  		}
>  	}
> @@ -7671,7 +7677,7 @@ static bool kick_one_cpu(s32 cpu, struct scx_sched_pcpu *pcpu, struct rq *this_r
>  	    !sched_class_above(cur_class, &ext_sched_class)) {
>  		if (cpumask_test_cpu(cpu, pcpu->cpus_to_preempt)) {
>  			if (cur_class == &ext_sched_class)
> -				rq->curr->scx.slice = 0;
> +				set_task_slice(rq->curr, 0);
>  			cpumask_clear_cpu(cpu, pcpu->cpus_to_preempt);
>  		}
>  
> @@ -8021,9 +8027,9 @@ __bpf_kfunc bool scx_bpf_dsq_insert___v2(struct task_struct *p, u64 dsq_id,
>  		return false;
>  
>  	if (slice)
> -		p->scx.slice = slice;
> +		set_task_slice(p, slice);
>  	else
> -		p->scx.slice = p->scx.slice ?: 1;
> +		set_task_slice(p, p->scx.slice ?: 1);
>  
>  	scx_dsq_insert_commit(sch, p, dsq_id, enq_flags);
>  
> @@ -8047,9 +8053,9 @@ static bool scx_dsq_insert_vtime(struct scx_sched *sch, struct task_struct *p,
>  		return false;
>  
>  	if (slice)
> -		p->scx.slice = slice;
> +		set_task_slice(p, slice);
>  	else
> -		p->scx.slice = p->scx.slice ?: 1;
> +		set_task_slice(p, p->scx.slice ?: 1);
>  
>  	p->scx.dsq_vtime = vtime;
>  
> @@ -8233,7 +8239,7 @@ static bool scx_dsq_move(struct bpf_iter_scx_dsq_kern *kit,
>  	if (kit->cursor.flags & __SCX_DSQ_ITER_HAS_VTIME)
>  		p->scx.dsq_vtime = kit->vtime;
>  	if (kit->cursor.flags & __SCX_DSQ_ITER_HAS_SLICE)
> -		p->scx.slice = kit->slice;
> +		set_task_slice(p, kit->slice);
>  
>  	/* execute move */
>  	locked_rq = move_task_between_dsqs(sch, p, enq_flags, src_dsq, dst_dsq);
> @@ -8628,7 +8634,7 @@ __bpf_kfunc bool scx_bpf_task_set_slice(struct task_struct *p, u64 slice,
>  	if (unlikely(!sch || !scx_task_on_sched(sch, p)))
>  		return false;
>  
> -	p->scx.slice = slice;
> +	set_task_slice(p, slice);
>  	return true;
>  }
>  
> -- 
> 2.54.0
>