linux-kernel - Re: CPU stuck due to the taprio hrtimer

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Message-ID: <2df10720-1790-48bd-a50c-4816260543b0@kylinos.cn>
Date: Mon, 2 Sep 2024 17:12:21 +0800
From: luyun <luyun@...inos.cn>
To: Vinicius Costa Gomes <vinicius.gomes@...el.com>, jhs@...atatu.com,
 xiyou.wangcong@...il.com, jiri@...nulli.us
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: CPU stuck due to the taprio hrtimer


在 2024/6/28 07:30, Vinicius Costa Gomes 写道:
> Yun Lu <luyun@...inos.cn> 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 <luyun@...inos.cn>
>> ---
>>   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.

Hello Vinicius,

May I ask is there a fix patch for this issue?

I test it on the latest kernel version,  and it still seems to cause CPU 
stuck.

As you mentioned, a better way would be to warn the user that the 
current time has jumped and cancel the hrtimer,

but I'm not sure how to warn the user, or just through printk?

Thanks and best regards.


>
>>   	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,