linux-kernel - [PATCH v4 2/3] sched_ext: Provide scx_bpf_task_acquire_remote

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Message-Id: <20250811212150.85759-3-christian.loehle@arm.com>
Date: Mon, 11 Aug 2025 22:21:49 +0100
From: Christian Loehle <christian.loehle@....com>
To: tj@...nel.org,
	arighi@...dia.com,
	void@...ifault.com
Cc: linux-kernel@...r.kernel.org,
	sched-ext@...ts.linux.dev,
	changwoo@...lia.com,
	hodgesd@...a.com,
	mingo@...hat.com,
	peterz@...radead.org,
	jake@...lion.co.uk,
	Christian Loehle <christian.loehle@....com>
Subject: [PATCH v4 2/3] sched_ext: Provide scx_bpf_task_acquire_remote_curr()

Provide scx_bpf_task_acquire_remote_curr() as a way for scx schedulers
to check the curr task of a remote rq without assuming its lock is
held.

Many scx schedulers make use of scx_bpf_cpu_rq() to check a remote curr
(e.g. to see if it should be preempted). This is problematic because
scx_bpf_cpu_rq() provides access to all fields of struct rq, most of
which aren't safe to use without holding the associated rq lock.

Signed-off-by: Christian Loehle <christian.loehle@....com>
---
 kernel/sched/ext.c                       | 24 ++++++++++++++++++++++++
 tools/sched_ext/include/scx/common.bpf.h |  1 +
 2 files changed, 25 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 14706c36ca83..ded4ace36090 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -7447,6 +7447,29 @@ __bpf_kfunc struct rq *scx_bpf_cpu_rq_locked(s32 cpu)
 	return rq;
 }
 
+/**
+ * scx_bpf_task_acquire_remote_curr - Fetch the curr task of a rq without
+ * acquiring its rq lock
+ * @cpu: CPU of the rq
+ *
+ * Increments the refcount of the task_struct which needs to be released using
+ * bpf_task_release().
+ */
+__bpf_kfunc struct task_struct *scx_bpf_task_acquire_remote_curr(s32 cpu)
+{
+	struct task_struct *p;
+
+	if (!kf_cpu_valid(cpu, NULL))
+		return NULL;
+
+	rcu_read_lock();
+	p = cpu_rq(cpu)->curr;
+	if (p)
+		p = refcount_inc_not_zero(&p->rcu_users) ? p : NULL;
+	rcu_read_unlock();
+	return p;
+}
+
 /**
  * scx_bpf_task_cgroup - Return the sched cgroup of a task
  * @p: task of interest
@@ -7612,6 +7635,7 @@ BTF_ID_FLAGS(func, scx_bpf_task_running, KF_RCU)
 BTF_ID_FLAGS(func, scx_bpf_task_cpu, KF_RCU)
 BTF_ID_FLAGS(func, scx_bpf_cpu_rq)
 BTF_ID_FLAGS(func, scx_bpf_cpu_rq_locked, KF_RET_NULL)
+BTF_ID_FLAGS(func, scx_bpf_task_acquire_remote_curr, KF_RET_NULL | KF_ACQUIRE)
 #ifdef CONFIG_CGROUP_SCHED
 BTF_ID_FLAGS(func, scx_bpf_task_cgroup, KF_RCU | KF_ACQUIRE)
 #endif
diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h
index 7451491347ed..2105cd0ee178 100644
--- a/tools/sched_ext/include/scx/common.bpf.h
+++ b/tools/sched_ext/include/scx/common.bpf.h
@@ -92,6 +92,7 @@ bool scx_bpf_task_running(const struct task_struct *p) __ksym;
 s32 scx_bpf_task_cpu(const struct task_struct *p) __ksym;
 struct rq *scx_bpf_cpu_rq(s32 cpu) __ksym;
 struct rq *scx_bpf_cpu_rq_locked(s32 cpu) __ksym;
+struct task_struct *scx_bpf_task_acquire_remote_curr(s32 cpu) __ksym;
 struct cgroup *scx_bpf_task_cgroup(struct task_struct *p) __ksym __weak;
 u64 scx_bpf_now(void) __ksym __weak;
 void scx_bpf_events(struct scx_event_stats *events, size_t events__sz) __ksym __weak;
-- 
2.34.1