From 44f5c8ec5b9ad8ed4ade08d727f803b2bb07f1c3 Mon Sep 17 00:00:00 2001 From: Ryan Newton Date: Wed, 15 Oct 2025 11:50:35 -0400 Subject: [PATCH] sched_ext: Add lockless peek operation for DSQs The builtin DSQ queue data structures are meant to be used by a wide range of different sched_ext schedulers with different demands on these data structures. They might be per-cpu with low-contention, or high-contention shared queues. Unfortunately, DSQs have a coarse-grained lock around the whole data structure. Without going all the way to a lock-free, more scalable implementation, a small step we can take to reduce lock contention is to allow a lockless, small-fixed-cost peek at the head of the queue. This change allows certain custom SCX schedulers to cheaply peek at queues, e.g. during load balancing, before locking them. But it represents a few extra memory operations to update the pointer each time the DSQ is modified, including a memory barrier on ARM so the write appears correctly ordered. This commit adds a first_task pointer field which is updated atomically when the DSQ is modified, and allows any thread to peek at the head of the queue without holding the lock. Signed-off-by: Ryan Newton Reviewed-by: Andrea Righi Reviewed-by: Christian Loehle Signed-off-by: Tejun Heo --- include/linux/sched/ext.h | 1 + kernel/sched/ext.c | 58 +++++++++++++++++++++++- tools/sched_ext/include/scx/common.bpf.h | 1 + tools/sched_ext/include/scx/compat.bpf.h | 18 ++++++++ 4 files changed, 76 insertions(+), 2 deletions(-) diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h index 9848aeab27864..4713f374acc0e 100644 --- a/include/linux/sched/ext.h +++ b/include/linux/sched/ext.h @@ -58,6 +58,7 @@ enum scx_dsq_id_flags { */ struct scx_dispatch_q { raw_spinlock_t lock; + struct task_struct __rcu *first_task; /* lockless peek at head */ struct list_head list; /* tasks in dispatch order */ struct rb_root priq; /* used to order by p->scx.dsq_vtime */ u32 nr; diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index 430749ce46ab3..f9c0888ef2796 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -965,8 +965,11 @@ static void dispatch_enqueue(struct scx_sched *sch, struct scx_dispatch_q *dsq, container_of(rbp, struct task_struct, scx.dsq_priq); list_add(&p->scx.dsq_list.node, &prev->scx.dsq_list.node); + /* first task unchanged - no update needed */ } else { list_add(&p->scx.dsq_list.node, &dsq->list); + /* not builtin and new task is at head - use fastpath */ + rcu_assign_pointer(dsq->first_task, p); } } else { /* a FIFO DSQ shouldn't be using PRIQ enqueuing */ @@ -974,10 +977,19 @@ static void dispatch_enqueue(struct scx_sched *sch, struct scx_dispatch_q *dsq, scx_error(sch, "DSQ ID 0x%016llx already had PRIQ-enqueued tasks", dsq->id); - if (enq_flags & (SCX_ENQ_HEAD | SCX_ENQ_PREEMPT)) + if (enq_flags & (SCX_ENQ_HEAD | SCX_ENQ_PREEMPT)) { list_add(&p->scx.dsq_list.node, &dsq->list); - else + /* new task inserted at head - use fastpath */ + if (!(dsq->id & SCX_DSQ_FLAG_BUILTIN)) + rcu_assign_pointer(dsq->first_task, p); + } else { + bool was_empty; + + was_empty = list_empty(&dsq->list); list_add_tail(&p->scx.dsq_list.node, &dsq->list); + if (was_empty && !(dsq->id & SCX_DSQ_FLAG_BUILTIN)) + rcu_assign_pointer(dsq->first_task, p); + } } /* seq records the order tasks are queued, used by BPF DSQ iterator */ @@ -1032,6 +1044,13 @@ static void task_unlink_from_dsq(struct task_struct *p, p->scx.dsq_flags &= ~SCX_TASK_DSQ_ON_PRIQ; } + if (!(dsq->id & SCX_DSQ_FLAG_BUILTIN) && dsq->first_task == p) { + struct task_struct *first_task; + + first_task = nldsq_next_task(dsq, NULL, false); + rcu_assign_pointer(dsq->first_task, first_task); + } + list_del_init(&p->scx.dsq_list.node); dsq_mod_nr(dsq, -1); } @@ -6292,6 +6311,40 @@ __bpf_kfunc void bpf_iter_scx_dsq_destroy(struct bpf_iter_scx_dsq *it) kit->dsq = NULL; } +/** + * scx_bpf_dsq_peek - Lockless peek at the first element. + * @dsq_id: DSQ to examine. + * + * Read the first element in the DSQ. This is semantically equivalent to using + * the DSQ iterator, but is lockfree. Of course, like any lockless operation, + * this provides only a point-in-time snapshot, and the contents may change + * by the time any subsequent locking operation reads the queue. + * + * Returns the pointer, or NULL indicates an empty queue OR internal error. + */ +__bpf_kfunc struct task_struct *scx_bpf_dsq_peek(u64 dsq_id) +{ + struct scx_sched *sch; + struct scx_dispatch_q *dsq; + + sch = rcu_dereference(scx_root); + if (unlikely(!sch)) + return NULL; + + if (unlikely(dsq_id & SCX_DSQ_FLAG_BUILTIN)) { + scx_error(sch, "peek disallowed on builtin DSQ 0x%llx", dsq_id); + return NULL; + } + + dsq = find_user_dsq(sch, dsq_id); + if (unlikely(!dsq)) { + scx_error(sch, "peek on non-existent DSQ 0x%llx", dsq_id); + return NULL; + } + + return rcu_dereference(dsq->first_task); +} + __bpf_kfunc_end_defs(); static s32 __bstr_format(struct scx_sched *sch, u64 *data_buf, char *line_buf, @@ -6851,6 +6904,7 @@ BTF_ID_FLAGS(func, scx_bpf_task_set_dsq_vtime, KF_RCU); BTF_ID_FLAGS(func, scx_bpf_kick_cpu) BTF_ID_FLAGS(func, scx_bpf_dsq_nr_queued) BTF_ID_FLAGS(func, scx_bpf_destroy_dsq) +BTF_ID_FLAGS(func, scx_bpf_dsq_peek, KF_RCU_PROTECTED | KF_RET_NULL) BTF_ID_FLAGS(func, bpf_iter_scx_dsq_new, KF_ITER_NEW | KF_RCU_PROTECTED) BTF_ID_FLAGS(func, bpf_iter_scx_dsq_next, KF_ITER_NEXT | KF_RET_NULL) BTF_ID_FLAGS(func, bpf_iter_scx_dsq_destroy, KF_ITER_DESTROY) diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h index eb3c99445cb3e..e65b1eb668ea5 100644 --- a/tools/sched_ext/include/scx/common.bpf.h +++ b/tools/sched_ext/include/scx/common.bpf.h @@ -74,6 +74,7 @@ u32 scx_bpf_reenqueue_local(void) __ksym; void scx_bpf_kick_cpu(s32 cpu, u64 flags) __ksym; s32 scx_bpf_dsq_nr_queued(u64 dsq_id) __ksym; void scx_bpf_destroy_dsq(u64 dsq_id) __ksym; +struct task_struct *scx_bpf_dsq_peek(u64 dsq_id) __ksym __weak; int bpf_iter_scx_dsq_new(struct bpf_iter_scx_dsq *it, u64 dsq_id, u64 flags) __ksym __weak; struct task_struct *bpf_iter_scx_dsq_next(struct bpf_iter_scx_dsq *it) __ksym __weak; void bpf_iter_scx_dsq_destroy(struct bpf_iter_scx_dsq *it) __ksym __weak; diff --git a/tools/sched_ext/include/scx/compat.bpf.h b/tools/sched_ext/include/scx/compat.bpf.h index e487c10b5e07f..619a16f0d39ac 100644 --- a/tools/sched_ext/include/scx/compat.bpf.h +++ b/tools/sched_ext/include/scx/compat.bpf.h @@ -26,6 +26,24 @@ int bpf_cpumask_populate(struct cpumask *dst, void *src, size_t src__sz) __ksym (bpf_ksym_exists(bpf_cpumask_populate) ? \ (bpf_cpumask_populate(cpumask, src, size__sz)) : -EOPNOTSUPP) +/* + * v6.19: Introduce lockless peek API for user DSQs. + * + * Preserve the following macro until v6.21. + */ +static inline struct task_struct *__COMPAT_scx_bpf_dsq_peek(u64 dsq_id) +{ + struct task_struct *p = NULL; + struct bpf_iter_scx_dsq it; + + if (bpf_ksym_exists(scx_bpf_dsq_peek)) + return scx_bpf_dsq_peek(dsq_id); + if (!bpf_iter_scx_dsq_new(&it, dsq_id, 0)) + p = bpf_iter_scx_dsq_next(&it); + bpf_iter_scx_dsq_destroy(&it); + return p; +} + /** * __COMPAT_is_enq_cpu_selected - Test if SCX_ENQ_CPU_SELECTED is on * in a compatible way. We will preserve this __COMPAT helper until v6.16. -- 2.47.3