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Mon, 09 Mar 2026 10:16:34 -0700 (PDT) Precedence: bulk X-Mailing-List: sched-ext@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Mon, 09 Mar 2026 13:16:33 -0400 Message-Id: To: "Emil Tsalapatis" , "Tejun Heo" , , Cc: , , Subject: Re: [PATCH 06/15] sched_ext: Convert deferred_reenq_locals from llist to regular list From: "Emil Tsalapatis" X-Mailer: aerc 0.20.1 References: <20260306190623.1076074-1-tj@kernel.org> <20260306190623.1076074-7-tj@kernel.org> In-Reply-To: On Mon Mar 9, 2026 at 1:12 PM EDT, Emil Tsalapatis wrote: > On Fri Mar 6, 2026 at 2:06 PM EST, Tejun Heo wrote: >> The deferred reenqueue local mechanism uses an llist (lockless list) for >> collecting schedulers that need their local DSQs re-enqueued. Convert to= a >> regular list protected by a raw_spinlock. >> >> The llist was used for its lockless properties, but the upcoming changes= to >> support remote reenqueue require more complex list operations that are >> difficult to implement correctly with lockless data structures. A spinlo= ck- >> protected regular list provides the necessary flexibility. >> >> Signed-off-by: Tejun Heo > > > Reviewed-by: Emil Tsalapatis > >> --- >> kernel/sched/ext.c | 57 ++++++++++++++++++++++++------------- >> kernel/sched/ext_internal.h | 2 +- >> kernel/sched/sched.h | 3 +- >> 3 files changed, 41 insertions(+), 21 deletions(-) >> >> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c >> index 1b6cd1e4f8b9..ffccaf04e34d 100644 >> --- a/kernel/sched/ext.c >> +++ b/kernel/sched/ext.c >> @@ -3640,23 +3640,37 @@ static u32 reenq_local(struct scx_sched *sch, st= ruct rq *rq) >> return nr_enqueued; >> } >> =20 >> -static void run_deferred(struct rq *rq) >> +static void process_deferred_reenq_locals(struct rq *rq) >> { >> - process_ddsp_deferred_locals(rq); >> - >> - if (!llist_empty(&rq->scx.deferred_reenq_locals)) { >> - struct llist_node *llist =3D >> - llist_del_all(&rq->scx.deferred_reenq_locals); >> - struct scx_sched_pcpu *pos, *next; >> + lockdep_assert_rq_held(rq); >> =20 >> - llist_for_each_entry_safe(pos, next, llist, >> - deferred_reenq_locals_node) { >> - init_llist_node(&pos->deferred_reenq_locals_node); >> - reenq_local(pos->sch, rq); >> + while (true) { >> + struct scx_sched *sch; >> + >> + scoped_guard (raw_spinlock, &rq->scx.deferred_reenq_lock) { >> + struct scx_sched_pcpu *sch_pcpu =3D >> + list_first_entry_or_null(&rq->scx.deferred_reenq_locals, >> + struct scx_sched_pcpu, >> + deferred_reenq_local_node); >> + if (!sch_pcpu) >> + return; >> + >> + sch =3D sch_pcpu->sch; > > While both scx and sch_pcpu aren't used in this patch, they are useful > for subsequent patches. > This comment was meant for the next patch in the series, sorry about that. The review tag still applies. >> + list_del_init(&sch_pcpu->deferred_reenq_local_node); >> } >> + >> + reenq_local(sch, rq); >> } >> } >> =20 >> +static void run_deferred(struct rq *rq) >> +{ >> + process_ddsp_deferred_locals(rq); >> + >> + if (!list_empty(&rq->scx.deferred_reenq_locals)) >> + process_deferred_reenq_locals(rq); >> +} >> + >> #ifdef CONFIG_NO_HZ_FULL >> bool scx_can_stop_tick(struct rq *rq) >> { >> @@ -4180,13 +4194,13 @@ static void scx_sched_free_rcu_work(struct work_= struct *work) >> =20 >> /* >> * $sch would have entered bypass mode before the RCU grace period. As >> - * that blocks new deferrals, all deferred_reenq_locals_node's must be >> + * that blocks new deferrals, all deferred_reenq_local_node's must be >> * off-list by now. >> */ >> for_each_possible_cpu(cpu) { >> struct scx_sched_pcpu *pcpu =3D per_cpu_ptr(sch->pcpu, cpu); >> =20 >> - WARN_ON_ONCE(llist_on_list(&pcpu->deferred_reenq_locals_node)); >> + WARN_ON_ONCE(!list_empty(&pcpu->deferred_reenq_local_node)); >> } >> =20 >> free_percpu(sch->pcpu); >> @@ -5799,7 +5813,7 @@ static struct scx_sched *scx_alloc_and_add_sched(s= truct sched_ext_ops *ops, >> struct scx_sched_pcpu *pcpu =3D per_cpu_ptr(sch->pcpu, cpu); >> =20 >> pcpu->sch =3D sch; >> - init_llist_node(&pcpu->deferred_reenq_locals_node); >> + INIT_LIST_HEAD(&pcpu->deferred_reenq_local_node); >> } >> =20 >> sch->helper =3D kthread_run_worker(0, "sched_ext_helper"); >> @@ -7126,7 +7140,8 @@ void __init init_sched_ext_class(void) >> BUG_ON(!zalloc_cpumask_var_node(&rq->scx.cpus_to_kick_if_idle, GFP_KE= RNEL, n)); >> BUG_ON(!zalloc_cpumask_var_node(&rq->scx.cpus_to_preempt, GFP_KERNEL,= n)); >> BUG_ON(!zalloc_cpumask_var_node(&rq->scx.cpus_to_wait, GFP_KERNEL, n)= ); >> - init_llist_head(&rq->scx.deferred_reenq_locals); >> + raw_spin_lock_init(&rq->scx.deferred_reenq_lock); >> + INIT_LIST_HEAD(&rq->scx.deferred_reenq_locals); >> rq->scx.deferred_irq_work =3D IRQ_WORK_INIT_HARD(deferred_irq_workfn)= ; >> rq->scx.kick_cpus_irq_work =3D IRQ_WORK_INIT_HARD(kick_cpus_irq_workf= n); >> =20 >> @@ -8358,7 +8373,6 @@ __bpf_kfunc void scx_bpf_reenqueue_local___v2(cons= t struct bpf_prog_aux *aux) >> unsigned long flags; >> struct scx_sched *sch; >> struct rq *rq; >> - struct llist_node *lnode; >> =20 >> raw_local_irq_save(flags); >> =20 >> @@ -8374,9 +8388,14 @@ __bpf_kfunc void scx_bpf_reenqueue_local___v2(con= st struct bpf_prog_aux *aux) >> goto out_irq_restore; >> =20 >> rq =3D this_rq(); >> - lnode =3D &this_cpu_ptr(sch->pcpu)->deferred_reenq_locals_node; >> - if (!llist_on_list(lnode)) >> - llist_add(lnode, &rq->scx.deferred_reenq_locals); >> + scoped_guard (raw_spinlock, &rq->scx.deferred_reenq_lock) { >> + struct scx_sched_pcpu *pcpu =3D this_cpu_ptr(sch->pcpu); >> + >> + if (list_empty(&pcpu->deferred_reenq_local_node)) >> + list_move_tail(&pcpu->deferred_reenq_local_node, >> + &rq->scx.deferred_reenq_locals); >> + } >> + >> schedule_deferred(rq); >> out_irq_restore: >> raw_local_irq_restore(flags); >> diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h >> index 9e5ebd00ea0c..80d40a9c5ad9 100644 >> --- a/kernel/sched/ext_internal.h >> +++ b/kernel/sched/ext_internal.h >> @@ -965,7 +965,7 @@ struct scx_sched_pcpu { >> */ >> struct scx_event_stats event_stats; >> =20 >> - struct llist_node deferred_reenq_locals_node; >> + struct list_head deferred_reenq_local_node; >> struct scx_dispatch_q bypass_dsq; >> #ifdef CONFIG_EXT_SUB_SCHED >> u32 bypass_host_seq; >> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h >> index ebe971d12cb8..0794852524e7 100644 >> --- a/kernel/sched/sched.h >> +++ b/kernel/sched/sched.h >> @@ -808,7 +808,8 @@ struct scx_rq { >> =20 >> struct task_struct *sub_dispatch_prev; >> =20 >> - struct llist_head deferred_reenq_locals; >> + raw_spinlock_t deferred_reenq_lock; >> + struct list_head deferred_reenq_locals; /* scheds requesting reenq of = local DSQ */ >> struct balance_callback deferred_bal_cb; >> struct irq_work deferred_irq_work; >> struct irq_work kick_cpus_irq_work;