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Message-ID: <aOTgMIGoeBuOGldz@gpd4>
Date: Tue, 7 Oct 2025 11:41:04 +0200
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
linux-kernel@...r.kernel.org, sched-ext@...ts.linux.dev
Subject: Re: [PATCH 4/4] sched_ext: Make scx_bpf_dsq_insert*() return bool
On Mon, Oct 06, 2025 at 03:51:47PM -1000, Tejun Heo wrote:
> In preparation for hierarchical schedulers, change scx_bpf_dsq_insert() and
> scx_bpf_dsq_insert_vtime() to return bool instead of void. With
> sub-schedulers, there will be no reliable way to guarantee a task is still
> owned by the sub-scheduler at insertion time (e.g., the task may have been
> migrated to another scheduler). The bool return value will enable
> sub-schedulers to detect and gracefully handle insertion failures.
>
> For the root scheduler, insertion failures will continue to trigger scheduler
> abort via scx_error(), so existing code doesn't need to check the return
> value. Backward compatibility is maintained through compat wrappers.
>
> Also update scx_bpf_dsq_move() documentation to clarify that it can return
> false for sub-schedulers when @dsq_id points to a disallowed local DSQ.
>
> Signed-off-by: Tejun Heo <tj@...nel.org>
> ---
...
> kernel/sched/ext.c | 45 ++++++++++++++++++------
> tools/sched_ext/include/scx/common.bpf.h | 3 +-
> tools/sched_ext/include/scx/compat.bpf.h | 23 ++++++++++--
> 3 files changed, 56 insertions(+), 15 deletions(-)
>
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index a34e731229de..399e53c8939c 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -5323,8 +5323,12 @@ __bpf_kfunc_start_defs();
> * exhaustion. If zero, the current residual slice is maintained. If
> * %SCX_SLICE_INF, @p never expires and the BPF scheduler must kick the CPU with
> * scx_bpf_kick_cpu() to trigger scheduling.
> + *
> + * Returns %true on successful insertion, %false on failure. On the root
> + * scheduler, %false return triggers scheduler abort and the caller doesn't need
> + * to check the return value.
> */
> -__bpf_kfunc void scx_bpf_dsq_insert(struct task_struct *p, u64 dsq_id, u64 slice,
> +__bpf_kfunc bool scx_bpf_dsq_insert(struct task_struct *p, u64 dsq_id, u64 slice,
> u64 enq_flags)
> {
> struct scx_sched *sch;
> @@ -5332,10 +5336,10 @@ __bpf_kfunc void scx_bpf_dsq_insert(struct task_struct *p, u64 dsq_id, u64 slice
> guard(rcu)();
> sch = rcu_dereference(scx_root);
> if (unlikely(!sch))
> - return;
> + return false;
>
> if (!scx_dsq_insert_preamble(sch, p, enq_flags))
> - return;
> + return false;
>
> if (slice)
> p->scx.slice = slice;
> @@ -5343,13 +5347,24 @@ __bpf_kfunc void scx_bpf_dsq_insert(struct task_struct *p, u64 dsq_id, u64 slice
> p->scx.slice = p->scx.slice ?: 1;
>
> scx_dsq_insert_commit(sch, p, dsq_id, enq_flags);
> +
> + return true;
> +}
> +
> +/*
> + * COMPAT: Will be removed in v6.23.
> + */
> +__bpf_kfunc void scx_bpf_dsq_insert___compat(struct task_struct *p, u64 dsq_id,
> + u64 slice, u64 enq_flags)
> +{
> + scx_bpf_dsq_insert(p, dsq_id, slice, enq_flags);
> }
>
> -static void scx_dsq_insert_vtime(struct scx_sched *sch, struct task_struct *p,
> +static bool scx_dsq_insert_vtime(struct scx_sched *sch, struct task_struct *p,
> u64 dsq_id, u64 slice, u64 vtime, u64 enq_flags)
> {
> if (!scx_dsq_insert_preamble(sch, p, enq_flags))
> - return;
> + return false;
>
> if (slice)
> p->scx.slice = slice;
> @@ -5359,6 +5374,8 @@ static void scx_dsq_insert_vtime(struct scx_sched *sch, struct task_struct *p,
> p->scx.dsq_vtime = vtime;
>
> scx_dsq_insert_commit(sch, p, dsq_id, enq_flags | SCX_ENQ_DSQ_PRIQ);
> +
> + return true;
> }
>
> struct scx_bpf_dsq_insert_vtime_args {
> @@ -5394,8 +5411,12 @@ struct scx_bpf_dsq_insert_vtime_args {
> * function must not be called on a DSQ which already has one or more FIFO tasks
> * queued and vice-versa. Also, the built-in DSQs (SCX_DSQ_LOCAL and
> * SCX_DSQ_GLOBAL) cannot be used as priority queues.
> + *
> + * Returns %true on successful insertion, %false on failure. On the root
> + * scheduler, %false return triggers scheduler abort and the caller doesn't need
> + * to check the return value.
> */
> -__bpf_kfunc void
> +__bpf_kfunc bool
> __scx_bpf_dsq_insert_vtime(struct task_struct *p,
> struct scx_bpf_dsq_insert_vtime_args *args)
> {
> @@ -5405,10 +5426,10 @@ __scx_bpf_dsq_insert_vtime(struct task_struct *p,
>
> sch = rcu_dereference(scx_root);
> if (unlikely(!sch))
> - return;
> + return false;
>
> - scx_dsq_insert_vtime(sch, p, args->dsq_id, args->slice, args->vtime,
> - args->enq_flags);
> + return scx_dsq_insert_vtime(sch, p, args->dsq_id, args->slice,
> + args->vtime, args->enq_flags);
> }
>
> /*
> @@ -5432,6 +5453,7 @@ __bpf_kfunc_end_defs();
>
> BTF_KFUNCS_START(scx_kfunc_ids_enqueue_dispatch)
> BTF_ID_FLAGS(func, scx_bpf_dsq_insert, KF_RCU)
> +BTF_ID_FLAGS(func, scx_bpf_dsq_insert___compat, KF_RCU)
> BTF_ID_FLAGS(func, __scx_bpf_dsq_insert_vtime, KF_RCU)
> BTF_ID_FLAGS(func, scx_bpf_dsq_insert_vtime, KF_RCU)
> BTF_KFUNCS_END(scx_kfunc_ids_enqueue_dispatch)
> @@ -5686,8 +5708,9 @@ __bpf_kfunc void scx_bpf_dsq_move_set_vtime(struct bpf_iter_scx_dsq *it__iter,
> * Can be called from ops.dispatch() or any BPF context which doesn't hold a rq
> * lock (e.g. BPF timers or SYSCALL programs).
> *
> - * Returns %true if @p has been consumed, %false if @p had already been consumed
> - * or dequeued.
> + * Returns %true if @p has been consumed, %false if @p had already been
> + * consumed, dequeued, or, for sub-scheds, @dsq_id points to a disallowed local
> + * DSQ.
> */
> __bpf_kfunc bool scx_bpf_dsq_move(struct bpf_iter_scx_dsq *it__iter,
> struct task_struct *p, u64 dsq_id,
> diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h
> index b1c2a0dde76e..522c90d0ced2 100644
> --- a/tools/sched_ext/include/scx/common.bpf.h
> +++ b/tools/sched_ext/include/scx/common.bpf.h
> @@ -62,8 +62,7 @@ s32 scx_bpf_create_dsq(u64 dsq_id, s32 node) __ksym;
> s32 scx_bpf_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, bool *is_idle) __ksym;
> s32 __scx_bpf_select_cpu_and(struct task_struct *p, const struct cpumask *cpus_allowed,
> struct scx_bpf_select_cpu_and_args *args) __ksym __weak;
> -void scx_bpf_dsq_insert(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flags) __ksym __weak;
> -void __scx_bpf_dsq_insert_vtime(struct task_struct *p, struct scx_bpf_dsq_insert_vtime_args *args) __ksym __weak;
> +bool __scx_bpf_dsq_insert_vtime(struct task_struct *p, struct scx_bpf_dsq_insert_vtime_args *args) __ksym __weak;
> u32 scx_bpf_dispatch_nr_slots(void) __ksym;
> void scx_bpf_dispatch_cancel(void) __ksym;
> bool scx_bpf_dsq_move_to_local(u64 dsq_id) __ksym __weak;
> diff --git a/tools/sched_ext/include/scx/compat.bpf.h b/tools/sched_ext/include/scx/compat.bpf.h
> index e172de696f99..33c26928f4e9 100644
> --- a/tools/sched_ext/include/scx/compat.bpf.h
> +++ b/tools/sched_ext/include/scx/compat.bpf.h
> @@ -196,7 +196,7 @@ scx_bpf_select_cpu_and(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
> * Inline wrapper that packs scalar arguments into a struct and calls
> * __scx_bpf_dsq_insert_vtime(). See __scx_bpf_dsq_insert_vtime() for details.
> */
> -static inline void
> +static inline bool
> scx_bpf_dsq_insert_vtime(struct task_struct *p, u64 dsq_id, u64 slice, u64 vtime,
> u64 enq_flags)
> {
> @@ -208,10 +208,29 @@ scx_bpf_dsq_insert_vtime(struct task_struct *p, u64 dsq_id, u64 slice, u64 vtime
> .enq_flags = enq_flags,
> };
>
> - __scx_bpf_dsq_insert_vtime(p, &args);
> + return __scx_bpf_dsq_insert_vtime(p, &args);
> } else {
> scx_bpf_dsq_insert_vtime___compat(p, dsq_id, slice, vtime,
> enq_flags);
> + return true;
> + }
> +}
> +
> +/*
> + * v6.19: scx_bpf_dsq_insert() now returns bool instead of void. Move
> + * scx_bpf_dsq_insert() decl to common.bpf.h and drop compat helper after v6.22.
> + */
> +bool scx_bpf_dsq_insert___new(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flags) __ksym __weak;
> +void scx_bpf_dsq_insert___compat(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flags) __ksym __weak;
> +
> +static inline bool
> +scx_bpf_dsq_insert(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flags)
> +{
> + if (bpf_ksym_exists(scx_bpf_dsq_insert___new)) {
I'm confused... where is scx_bpf_dsq_insert___new() defined?
> + return scx_bpf_dsq_insert___new(p, dsq_id, slice, enq_flags);
> + } else {
> + scx_bpf_dsq_insert___compat(p, dsq_id, slice, enq_flags);
> + return true;
> }
> }
>
> --
> 2.51.0
>
Thanks,
-Andrea
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