Re: [PATCH] sched/deadline: Don't count nr_running twice for dl_server proxy tasks

[Juri Lelli <juri.lelli@redhat.com>]

Hello,

On 27/06/25 11:54, Yicong Yang wrote:
> From: Yicong Yang <yangyicong@hisilicon.com>
> 
> On CPU offline the kernel stalled with below call trace:
> 
>   INFO: task kworker/0:1:11 blocked for more than 120 seconds.
>         Tainted: G           O        6.15.0-rc4+ #1
>   "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>   task:kworker/0:1     state:D stack:0     pid:11    tgid:11    ppid:2   task_flags:0x4208060 flags:0x00000008
>   Workqueue: events vmstat_shepherd
>   Call trace:
>    __switch_to+0x118/0x188 (T)
>    __schedule+0x31c/0x1300
>    schedule+0x3c/0x120
>    percpu_rwsem_wait+0x12c/0x1b0
>    __percpu_down_read+0x78/0x188
>    cpus_read_lock+0xc4/0xe8
>    vmstat_shepherd+0x30/0x138
>    process_one_work+0x154/0x3c8
>    worker_thread+0x2e8/0x400
>    kthread+0x154/0x230
>    ret_from_fork+0x10/0x20
>   INFO: task kworker/1:1:116 blocked for more than 120 seconds.
>         Tainted: G           O        6.15.0-rc4+ #1
>   "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>   task:kworker/1:1     state:D stack:0     pid:116   tgid:116   ppid:2   task_flags:0x4208060 flags:0x00000008
>   Workqueue: events work_for_cpu_fn
>   Call trace:
>    __switch_to+0x118/0x188 (T)
>    __schedule+0x31c/0x1300
>    schedule+0x3c/0x120
>    schedule_timeout+0x10c/0x120
>    __wait_for_common+0xc4/0x1b8
>    wait_for_completion+0x28/0x40
>    cpuhp_kick_ap_work+0x114/0x3c8
>    _cpu_down+0x130/0x4b8
>    __cpu_down_maps_locked+0x20/0x38
>    work_for_cpu_fn+0x24/0x40
>    process_one_work+0x154/0x3c8
>    worker_thread+0x2e8/0x400
>    kthread+0x154/0x230
>    ret_from_fork+0x10/0x20
> 
> cpuhp hold the cpu hotplug lock endless and stalled vmstat_shepherd.
> This is because we count nr_running twice on cpuhp enqueuing and failed
> the wait condition of cpuhp:
> 
> enqueue_task_fair() // pick cpuhp from idle, rq->nr_running = 0
>   dl_server_start()
>     [...]
>     add_nr_running() // rq->nr_running = 1
>   add_nr_running() // rq->nr_running = 2
> [switch to cpuhp, waiting on balance_hotplug_wait()]
> rcuwait_wait_event(rq->nr_running == 1 && ...) // failed, rq->nr_running=2
>   schedule() // wait again
> 
> This doesn't make sense to count one single task twice on
> rq->nr_running. So fix this.
> 
> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> ---
>  kernel/sched/deadline.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index ad45a8fea245..59fb178762ee 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1894,7 +1894,9 @@ void inc_dl_tasks(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
>  	u64 deadline = dl_se->deadline;
>  
>  	dl_rq->dl_nr_running++;
> -	add_nr_running(rq_of_dl_rq(dl_rq), 1);
> +
> +	if (!dl_server(dl_se))
> +		add_nr_running(rq_of_dl_rq(dl_rq), 1);
>  
>  	inc_dl_deadline(dl_rq, deadline);
>  }
> @@ -1904,7 +1906,9 @@ void dec_dl_tasks(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
>  {
>  	WARN_ON(!dl_rq->dl_nr_running);
>  	dl_rq->dl_nr_running--;
> -	sub_nr_running(rq_of_dl_rq(dl_rq), 1);
> +
> +	if (!dl_server(dl_se))
> +		sub_nr_running(rq_of_dl_rq(dl_rq), 1);
>  
>  	dec_dl_deadline(dl_rq, dl_se->deadline);
>  }

This seems to make sense to me. Thanks for the analysis and the patch.

Peter, what do you think?

Thanks,
Juri