Subject: Re: CPU stuck due to the taprio hrtimer

From: Vinicius Costa Gomes <[email-masked]>
Date: Thu, 27 Jun 2024 16:30:30 -0700

Yun Lu <[email-masked]> writes:

> Hello,
>
> When I run a taprio test program on the latest kernel(v6.10-rc4), CPU stuck
> is detected immediately, and the stack shows that CPU is stuck on taprio
> hrtimer.
>
> The reproducer program link:
> https://github.com/xyyluyun/taprio_test/blob/main/taprio_test.c
> gcc taprio_test.c -static -o taprio_test
>
> In this program, start the taprio hrtimer which clockid is set to REALTIME, and
> then adjust the system time by a significant value backwards. Thus, CPU will enter
> an infinite loop in the__hrtimer_run_queues function, getting stuck and unable to
> exit or respond to any interrupts.
>
> I have tried to avoid this problem by apllying the following patch, and it does work.
> But I am not sure if this can be the final solution?
>
> Thanks.
>
> Signed-off-by: Yun Lu <[email-masked]>
> ---
>  net/sched/sch_taprio.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
>
> diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> index a0d54b422186..2ff8d34bdbac 100644
> --- a/net/sched/sch_taprio.c
> +++ b/net/sched/sch_taprio.c
> @@ -104,6 +104,7 @@ struct taprio_sched {
>  	u32 max_sdu[TC_MAX_QUEUE]; /* save info from the user */
>  	u32 fp[TC_QOPT_MAX_QUEUE]; /* only for dump and offloading */
>  	u32 txtime_delay;
> +	ktime_t offset;
>  };
>  
>  struct __tc_taprio_qopt_offload {
> @@ -170,6 +171,19 @@ static ktime_t sched_base_time(const struct sched_gate_list *sched)
>  	return ns_to_ktime(sched->base_time);
>  }
>  
> +static ktime_t taprio_get_offset(const struct taprio_sched *q)
> +{
> +	enum tk_offsets tk_offset = READ_ONCE(q->tk_offset);
> +	ktime_t time = ktime_get();
> +
> +	switch (tk_offset) {
> +	case TK_OFFS_MAX:
> +		return 0;
> +	default:
> +		return ktime_sub_ns(ktime_mono_to_any(time, tk_offset), time);
> +	}
> +}
> +
>  static ktime_t taprio_mono_to_any(const struct taprio_sched *q, ktime_t mono)
>  {
>  	/* This pairs with WRITE_ONCE() in taprio_parse_clockid() */
> @@ -918,6 +932,7 @@ static enum hrtimer_restart advance_sched(struct hrtimer *timer)
>  	int num_tc = netdev_get_num_tc(dev);
>  	struct sched_entry *entry, *next;
>  	struct Qdisc *sch = q->root;
> +	ktime_t now_offset = taprio_get_offset(q);
>  	ktime_t end_time;
>  	int tc;
>  
> @@ -957,6 +972,14 @@ static enum hrtimer_restart advance_sched(struct hrtimer *timer)
>  	end_time = ktime_add_ns(entry->end_time, next->interval);
>  	end_time = min_t(ktime_t, end_time, oper->cycle_end_time);
>  
> +	if (q->offset != now_offset) {
> +		ktime_t diff = ktime_sub_ns(now_offset, q->offset);
> +
> +		end_time = ktime_add_ns(end_time, diff);
> +		oper->cycle_end_time = ktime_add_ns(oper->cycle_end_time, diff);
> +		q->offset = now_offset;
> +	}
> +

I think what we should do here is a bit different. Let me try to explain
what I have in mind with some context.

A bit of context: The idea of taprio is to enforce "TSN" traffic
schedules, these schedules require time synchronization, for example via
PTP, and in those cases, time jumps are not expected or a sign that
something is wrong.

In my mind, a time jump, specially a big one, kind of invalidates the
schedule, as the schedule is based on an absolute time value (the
base_time), and when time jumps that reference in time is lost.

BUT making the user's system unresponsive is a bug, a big one, as if
this happens in the real world, the user will be unable to investigate
what made the system have so big a time correction.

So my idea is to warn the user that the time jumped, say that the user
needs to reconfigure the schedule, as it is now invalid, and disable the
schedule.

Does this make sense?

Ah, and thanks for the report.

>  	for (tc = 0; tc < num_tc; tc++) {
>  		if (next->gate_duration[tc] == oper->cycle_time)
>  			next->gate_close_time[tc] = KTIME_MAX;
> @@ -1210,6 +1233,7 @@ static int taprio_get_start_time(struct Qdisc *sch,
>  
>  	base = sched_base_time(sched);
>  	now = taprio_get_time(q);
> +	q->offset = taprio_get_offset(q);
>  
>  	if (ktime_after(base, now)) {
>  		*start = base;
> -- 
> 2.34.1
>

Cheers,
-- 
Vinicius