[PATCH v3 1/2] sched_ext: Add lockless peek operation for DSQs

Ryan Newton
2025-10-06T17:04:13Z

From: Ryan Newton <newton@meta.com>

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 <newton@meta.com>
---
include/linux/sched/ext.h | 1 +
kernel/sched/ext.c | 54 +++++++++++++++++++++++-
tools/sched_ext/include/scx/common.bpf.h | 1 +
tools/sched_ext/include/scx/compat.bpf.h | 19 +++++++++
4 files changed, 73 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index d82b7a9b0658..81478d4ae782 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 2b0e88206d07..6d3537e65001 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -944,8 +944,11 @@ 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 */
@@ -953,10 +956,19 @@ 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 */
@@ -1011,6 +1023,13 @@ 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);
}
@@ -6084,6 +6103,36 @@ __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.
+ *
+ * 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,
@@ -6641,6 +6690,7 @@ BTF_KFUNCS_START(scx_kfunc_ids_any)
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 06e2551033cb..fbf3e7f9526c 100644
--- a/tools/sched_ext/include/scx/common.bpf.h
+++ b/tools/sched_ext/include/scx/common.bpf.h
@@ -75,6 +75,7 @@ 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 dd9144624dc9..97b10c184b2c 100644
--- a/tools/sched_ext/include/scx/compat.bpf.h
+++ b/tools/sched_ext/include/scx/compat.bpf.h
@@ -130,6 +130,25 @@ int bpf_cpumask_populate(struct cpumask *dst, void *src, size_t src__sz) __ksym
false; \
})

+
+/*
+ * 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.
--
2.51.0