Re: [PATCH] sched/stats: TASK_IDLE task bypass the block_starts time

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jun 20, 2025 at 11:14:50AM +0800, Olice Zou wrote:
> For TASK_IDLE task, we not should record the block_starts, it is
> not real TASK_UNINTERRUPTIBLE task.

Why, I mean it is still blocked, right?

> It is easy to find this problem in a idle machine as followe:
> 
> bpftrace -e 'tracepoint:sched:sched_stat_blocked {  \
>     if (args->delay > 1000000)  \
>     {  \
> 	printf("%s %d\n", args->comm, args->delay);  \
> 	print(kstack());  \
>     }  \
> }
> 
> rcu_preempt 3881764
>         __update_stats_enqueue_sleeper+604
>         __update_stats_enqueue_sleeper+604
>         enqueue_entity+1014
>         enqueue_task_fair+156
>         activate_task+109
>         ttwu_do_activate+111
>         try_to_wake_up+615
>         wake_up_process+25
>         process_timeout+22
>         call_timer_fn+44
>         run_timer_softirq+1100
>         handle_softirqs+178
>         irq_exit_rcu+113
>         sysvec_apic_timer_interrupt+132
>         asm_sysvec_apic_timer_interrupt+31
>         pv_native_safe_halt+15
>         arch_cpu_idle+13
>         default_idle_call+48
>         do_idle+516
>         cpu_startup_entry+49
>         start_secondary+280
>         secondary_startup_64_no_verify+404

Not sure what I'm looking at there. What is the problem?

> Signed-off-by: Olice Zou <olicezou@tencent.com>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a85539df75a5..e473e3244dda 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1285,7 +1285,7 @@ update_stats_dequeue_fair(struct cfs_rq *cfs_rq, struct sched_entity *se, int fl
>  		if (state & TASK_INTERRUPTIBLE)
>  			__schedstat_set(tsk->stats.sleep_start,
>  				      rq_clock(rq_of(cfs_rq)));
> -		if (state & TASK_UNINTERRUPTIBLE)
> +		if (state != TASK_IDLE && (state & TASK_UNINTERRUPTIBLE))
>  			__schedstat_set(tsk->stats.block_start,
>  				      rq_clock(rq_of(cfs_rq)));
>  	}
> -- 
> 2.25.1
>