Re: [PATCH v2] tools/sched_ext: scx_central: start timer from central dispatch

[Zhao Mengmeng]

On 3/27/26 10:48, Zhao Mengmeng wrote:
> 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, loading
> scx_central can fail 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)
> 
> To fix it,
> - Defer bpf_timer_start() to the first dispatch on the central CPU.
> - In central_init(), initialize the BPF timer and kick central CPU, so
> that we're guaranteed to enter the dispatch path on the central CPU
> immediately.
> - sched_setaffinity() is unnecessary now, so remote it.

remote --> remove, will resend it soon.
> 
> Suggested-by: Tejun Heo <tj@xxxxxxxxxx>
> Signed-off-by: Zhao Mengmeng <zhaomengmeng@xxxxxxxxxx>
> ---
>  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(&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)
> 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)