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Message-Id: <20250901132605.2282650-4-christian.loehle@arm.com>
Date: Mon, 1 Sep 2025 14:26:03 +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 v5 2/3] sched_ext: Introduce 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 | 14 ++++++++++++++
tools/sched_ext/include/scx/common.bpf.h | 1 +
2 files changed, 15 insertions(+)
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 9fcc310d85d5..e242a2520f06 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -7452,6 +7452,19 @@ __bpf_kfunc struct rq *scx_bpf_cpu_rq_locked(void)
return rq;
}
+/**
+ * scx_bpf_remote_curr - Return remote CPU's curr task
+ * @cpu: CPU of interest
+ *
+ * Callers must hold RCU read lock (KF_RCU).
+ */
+__bpf_kfunc struct task_struct *scx_bpf_remote_curr(s32 cpu)
+{
+ if (!kf_cpu_valid(cpu, NULL))
+ return NULL;
+ return READ_ONCE(cpu_rq(cpu)->curr);
+}
+
/**
* scx_bpf_task_cgroup - Return the sched cgroup of a task
* @p: task of interest
@@ -7617,6 +7630,7 @@ 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_remote_curr, KF_RET_NULL | KF_RCU)
#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 f5be06c93359..dd3d94256c10 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(void) __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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