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Message-Id: <20250804112743.711816-3-christian.loehle@arm.com>
Date: Mon, 4 Aug 2025 12:27:42 +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,
Christian Loehle <christian.loehle@....com>
Subject: [PATCH v2 2/3] sched_ext: Provide scx_bpf_remote_curr()
Provide scx_bpf_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 3ea3f0f18030..1d9d9cbed0aa 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -7426,6 +7426,29 @@ __bpf_kfunc struct rq *scx_bpf_cpu_rq(s32 cpu)
return cpu_rq(cpu);
}
+struct task_struct *bpf_task_acquire(struct task_struct *p);
+
+/**
+ * scx_bpf_remote_curr - Fetch the curr 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 later.
+ */
+__bpf_kfunc struct task_struct *scx_bpf_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 = bpf_task_acquire(p);
+ rcu_read_unlock();
+ return p;
+}
+
/**
* scx_bpf_task_cgroup - Return the sched cgroup of a task
* @p: task of interest
@@ -7590,6 +7613,7 @@ BTF_ID_FLAGS(func, scx_bpf_put_cpumask, KF_RELEASE)
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, KF_RET_NULL)
+BTF_ID_FLAGS(func, scx_bpf_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 d4e21558e982..e5d4ef124532 100644
--- a/tools/sched_ext/include/scx/common.bpf.h
+++ b/tools/sched_ext/include/scx/common.bpf.h
@@ -91,6 +91,7 @@ s32 scx_bpf_pick_any_cpu(const cpumask_t *cpus_allowed, u64 flags) __ksym;
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 task_struct *scx_bpf_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
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