Re: [PATCH v2 05/21] sched/deadline: Do not BUG() on a deadline task with no active CPU allowed

[Juri Lelli]

Hello,

On 04/11/26 16:11, Mathieu Desnoyers wrote:
> dl_task_root_domain_cpu() finds the root domain that holds a deadline
> task's bandwidth from the first active CPU of the task's mask, and
> BUG()s when the mask has none. A mask can have none:
> 
>  - a task boosted into the deadline class by priority inheritance keeps
>    the affinity of its own policy, which may allow one CPU alone, or a
>    cpuset of one. dl_task_needs_bw_move() and set_cpus_allowed_dl()
>    take it for a deadline task, dl_task(p) being true, and look the root
>    domain up from the mask they are replacing, which allows that CPU,
>    inactive by then: the cpuset code updates the masks of a cpuset's
>    tasks once the CPU is inactive, and select_fallback_rq() widens the
>    mask of a task it pushes off that CPU;
>  - with admission control off (sched_rt_runtime_us at -1), a deadline
>    task may be pinned to fewer CPUs than its root domain spans, and
>    those may all go offline.
> 
> Both take the kernel down on the first try, in 8-CPU VMs of v7.4-rc1
> with the four deadline fixes of sched/urgent. The priority inheritance
            ^^^
Not sure how much sense will this info make when reading this changelog
in the future. :)

> one, which asks for no unusual configuration, through the wakeup of the
> boosted owner:
> 
>   kernel BUG at kernel/sched/deadline.c:3340!
>   CPU: 7 UID: 0 PID: 0 Comm: swapper/7
>   RIP: 0010:set_cpus_allowed_dl+0xf0/0x100
>   Call Trace:
>    <IRQ>
>    set_cpus_allowed_force+0x9f/0xf0
>    cpuset_cpus_allowed_fallback+0x12b/0x1d0
>    select_fallback_rq+0x238/0x340
>    try_to_wake_up+0x2a0/0xb70
> 
> Fall back to the CPU the task last ran on, which these callers used
> before the commit this one fixes. Its root domain may have no active CPU
> left: the previous patch has __dl_sub() and __dl_add() take that. Ten
> rounds of each case leave no warning with it.
> 
> Fixes: b4a61fdc19c5 ("sched/deadline: Fix stale root domain in cpuset bandwidth transfer")
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> ---
>  kernel/sched/deadline.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index dadda1a528ec..30a52e42c26d 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -3338,13 +3338,19 @@ static void task_woken_dl(struct rq *rq, struct task_struct *p)
>   * current cpus_mask (not yet updated to the new mask) to find the
>   * actual root domain. Use cpus_mask rather than cpus_ptr, which
>   * may be temporarily narrowed by migrate_disable().
> + *
> + * The mask has no active CPU left when the CPUs it allows went offline,
> + * which the affinity of a deadline task allows with admission control
> + * off, and that of a task boosted into the deadline class by priority
> + * inheritance always: fall back to the CPU the task last ran on.
>   */
>  static unsigned int dl_task_root_domain_cpu(struct task_struct *p)
>  {
>  	unsigned int cpu;
>  
>  	cpu = cpumask_first_and(&p->cpus_mask, cpu_active_mask);
> -	BUG_ON(cpu >= nr_cpu_ids);
> +	if (cpu >= nr_cpu_ids)
> +		cpu = task_cpu(p);
>  	return cpu;
>  }

At least (worst case) we fall back to how things were before my change
introduced the regression.

Acked-by: Juri Lelli <juri.lelli@xxxxxxxxxx>

Thanks,
Juri