[PATCH RESEND v2] scx_central: Defer timer start to central dispatch to fix init error
From: Zhao Mengmeng
Date: Fri Mar 27 2026 - 02:23:18 EST
From: Zhao Mengmeng <zhaomengmeng@xxxxxxxxxx>
scx_central currently assumes that ops.init() runs on the selected
central CPU and aborts otherwise. This is no longer true, as ops.init()
is invoked from the scx_enable_helper thread, which can run on any
CPU.
As a result, sched_setaffinity() from userspace doesn't work, causing
scx_central to fail when loading with:
[ 1985.319942] sched_ext: central: scx_central.bpf.c:314: init from non-central CPU
[ 1985.320317] scx_exit+0xa3/0xd0
[ 1985.320535] scx_bpf_error_bstr+0xbd/0x220
[ 1985.320840] bpf_prog_3a445a8163fa8149_central_init+0x103/0x1ba
[ 1985.321073] bpf__sched_ext_ops_init+0x40/0xa8
[ 1985.321286] scx_root_enable_workfn+0x507/0x1650
[ 1985.321461] kthread_worker_fn+0x260/0x940
[ 1985.321745] kthread+0x303/0x3e0
[ 1985.321901] ret_from_fork+0x589/0x7d0
[ 1985.322065] ret_from_fork_asm+0x1a/0x30
DEBUG DUMP
===================================================================
central: root
scx_enable_help[134] triggered exit kind 1025:
scx_bpf_error (scx_central.bpf.c:314: init from non-central CPU)
Fix this by:
- Defer bpf_timer_start() to the first dispatch on the central CPU.
- Initialize the BPF timer in central_init() and kick the central CPU
to guarantee entering the dispatch path on the central CPU immediately.
- Remove the unnecessary sched_setaffinity() call in userspace.
Suggested-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Zhao Mengmeng <zhaomengmeng@xxxxxxxxxx>
---
V2:
- Remove sched_setaffinity() call and kick central cpu at ops.init() as
suggested by Tejun.
- Refactor the commit message and header for better clarity.
---
tools/sched_ext/scx_central.bpf.c | 62 +++++++++++++++++++++----------
tools/sched_ext/scx_central.c | 24 ------------
2 files changed, 42 insertions(+), 44 deletions(-)
diff --git a/tools/sched_ext/scx_central.bpf.c b/tools/sched_ext/scx_central.bpf.c
index 399e8d3f8bec..4efcce099bd5 100644
--- a/tools/sched_ext/scx_central.bpf.c
+++ b/tools/sched_ext/scx_central.bpf.c
@@ -60,6 +60,7 @@ const volatile u32 nr_cpu_ids = 1; /* !0 for veristat, set during init */
const volatile u64 slice_ns;
bool timer_pinned = true;
+bool timer_started;
u64 nr_total, nr_locals, nr_queued, nr_lost_pids;
u64 nr_timers, nr_dispatches, nr_mismatches, nr_retries;
u64 nr_overflows;
@@ -179,9 +180,47 @@ static bool dispatch_to_cpu(s32 cpu)
return false;
}
+static void start_central_timer(void)
+{
+ struct bpf_timer *timer;
+ u32 key = 0;
+ int ret;
+
+ if (likely(timer_started))
+ return;
+
+ timer = bpf_map_lookup_elem(¢ral_timer, &key);
+ if (!timer) {
+ scx_bpf_error("failed to lookup central timer");
+ return;
+ }
+
+ ret = bpf_timer_start(timer, TIMER_INTERVAL_NS, BPF_F_TIMER_CPU_PIN);
+ /*
+ * BPF_F_TIMER_CPU_PIN is pretty new (>=6.7). If we're running in a
+ * kernel which doesn't have it, bpf_timer_start() will return -EINVAL.
+ * Retry without the PIN. This would be the perfect use case for
+ * bpf_core_enum_value_exists() but the enum type doesn't have a name
+ * and can't be used with bpf_core_enum_value_exists(). Oh well...
+ */
+ if (ret == -EINVAL) {
+ timer_pinned = false;
+ ret = bpf_timer_start(timer, TIMER_INTERVAL_NS, 0);
+ }
+
+ if (ret) {
+ scx_bpf_error("bpf_timer_start failed (%d)", ret);
+ return;
+ }
+
+ timer_started = true;
+}
+
void BPF_STRUCT_OPS(central_dispatch, s32 cpu, struct task_struct *prev)
{
if (cpu == central_cpu) {
+ start_central_timer();
+
/* dispatch for all other CPUs first */
__sync_fetch_and_add(&nr_dispatches, 1);
@@ -310,29 +349,12 @@ int BPF_STRUCT_OPS_SLEEPABLE(central_init)
if (!timer)
return -ESRCH;
- if (bpf_get_smp_processor_id() != central_cpu) {
- scx_bpf_error("init from non-central CPU");
- return -EINVAL;
- }
-
bpf_timer_init(timer, ¢ral_timer, CLOCK_MONOTONIC);
bpf_timer_set_callback(timer, central_timerfn);
- ret = bpf_timer_start(timer, TIMER_INTERVAL_NS, BPF_F_TIMER_CPU_PIN);
- /*
- * BPF_F_TIMER_CPU_PIN is pretty new (>=6.7). If we're running in a
- * kernel which doesn't have it, bpf_timer_start() will return -EINVAL.
- * Retry without the PIN. This would be the perfect use case for
- * bpf_core_enum_value_exists() but the enum type doesn't have a name
- * and can't be used with bpf_core_enum_value_exists(). Oh well...
- */
- if (ret == -EINVAL) {
- timer_pinned = false;
- ret = bpf_timer_start(timer, TIMER_INTERVAL_NS, 0);
- }
- if (ret)
- scx_bpf_error("bpf_timer_start failed (%d)", ret);
- return ret;
+ scx_bpf_kick_cpu(central_cpu, 0);
+
+ return 0;
}
void BPF_STRUCT_OPS(central_exit, struct scx_exit_info *ei)
diff --git a/tools/sched_ext/scx_central.c b/tools/sched_ext/scx_central.c
index fd4c0eaa4326..4a72df39500d 100644
--- a/tools/sched_ext/scx_central.c
+++ b/tools/sched_ext/scx_central.c
@@ -5,7 +5,6 @@
* Copyright (c) 2022 David Vernet <dvernet@xxxxxxxx>
*/
#define _GNU_SOURCE
-#include <sched.h>
#include <stdio.h>
#include <unistd.h>
#include <inttypes.h>
@@ -49,8 +48,6 @@ int main(int argc, char **argv)
struct bpf_link *link;
__u64 seq = 0, ecode;
__s32 opt;
- cpu_set_t *cpuset;
- size_t cpuset_size;
libbpf_set_print(libbpf_print_fn);
signal(SIGINT, sigint_handler);
@@ -96,27 +93,6 @@ int main(int argc, char **argv)
SCX_OPS_LOAD(skel, central_ops, scx_central, uei);
- /*
- * Affinitize the loading thread to the central CPU, as:
- * - That's where the BPF timer is first invoked in the BPF program.
- * - We probably don't want this user space component to take up a core
- * from a task that would benefit from avoiding preemption on one of
- * the tickless cores.
- *
- * Until BPF supports pinning the timer, it's not guaranteed that it
- * will always be invoked on the central CPU. In practice, this
- * suffices the majority of the time.
- */
- cpuset = CPU_ALLOC(skel->rodata->nr_cpu_ids);
- SCX_BUG_ON(!cpuset, "Failed to allocate cpuset");
- cpuset_size = CPU_ALLOC_SIZE(skel->rodata->nr_cpu_ids);
- CPU_ZERO_S(cpuset_size, cpuset);
- CPU_SET_S(skel->rodata->central_cpu, cpuset_size, cpuset);
- SCX_BUG_ON(sched_setaffinity(0, cpuset_size, cpuset),
- "Failed to affinitize to central CPU %d (max %d)",
- skel->rodata->central_cpu, skel->rodata->nr_cpu_ids - 1);
- CPU_FREE(cpuset);
-
link = SCX_OPS_ATTACH(skel, central_ops, scx_central);
if (!skel->data->timer_pinned)
--
2.43.0