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

[Tejun Heo]

Hello, Zhao.

On Thu, Mar 26, 2026 at 05:15:23PM +0800, 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, initialize the BPF timer from ops.init(), but defer
> bpf_timer_start() to the first dispatch on the central CPU. This preserves
> the requirement that the timer be started from the central CPU when
> BPF_F_TIMER_CPU_PIN is used, without depending on the CPU affinity of
> the enable path.

Can you do the followings please?

- Remove now unnecessary sched_setaffinity() call.

- Do scx_bpf_kick_cpu(central_cpu, 0) so that we're guaranteed to enter the
  dispatch path on the central CPU immediately.

Thanks.

-- 
tejun