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scx_central: Defer timer start to central dispatch to fix init error

Browse Source 
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@kernel.org>
Signed-off-by: Zhao Mengmeng <zhaomengmeng@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Zhao Mengmeng
2026-03-27 14:17:57 +08:00
committed by Tejun Heo
parent c6f99d0ecc
commit d6edb15ad9
2 changed files with 42 additions and 44 deletions
Download Patch File Download Diff File Expand all files Collapse all files

62
tools/sched_ext/scx_central.bpf.c
View File

@@ -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(&central_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, &central_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)

24
tools/sched_ext/scx_central.c
View File

@@ -5,7 +5,6 @@
* Copyright (c) 2022 David Vernet <dvernet@meta.com>
*/
#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)