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Message-ID: <CABFh=a5QHpNh8CAmw5-REJuz1Qzg0wFEp1ZeArM9O1UCN5ebtQ@mail.gmail.com>
Date: Tue, 7 Oct 2025 11:03:04 -0400
From: Emil Tsalapatis <linux-lists@...alapatis.com>
To: Andrea Righi <arighi@...dia.com>
Cc: Tejun Heo <tj@...nel.org>, 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 Tue, Oct 7, 2025 at 5:41 AM Andrea Righi <arighi@...dia.com> wrote:
>
> 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?
>
CO:RE relocation ignores suffixes if there is no match, so
scx_bpf_dsq_insert___new() defaults to scx_bpf_dsq_insert() in systems
with this patch. When I tested it the symbol resolved to
scx_bpf_dsq_insert(). We're not really matching the name, we're
matching the signature and trying to find a kfunc with the the
scx_bpf_dsq_insert prefix in its name.
> > + 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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