Subject: Problem about CPU stalling in hrtimer_intterrupts()

From: [email-masked] (Yang Yingliang)
Date: Thu, 22 Oct 2015 21:43:05 +0800

On 2015/10/22 18:25, Thomas Gleixner wrote:
> On Thu, 22 Oct 2015, Ding Tianhong wrote:
>> On 2015/10/22 15:43, Thomas Gleixner wrote:
>>>> Jan 01 00:03:32 Linux kernel: i:0 basenow.tv64:4809284991830
>>>> hrtimer_get_softexpires_tv64(timer):4440120000000 ccpu0
>>>> timer:ffffffdffdec6138, timer->function:ffffffc000129b84
>>>> Jan 01 00:03:32 Linux kernel: i:0 basenow.tv64:4809284991830
>>>> hrtimer_get_softexpires_tv64(timer):4440120000000 ccpu0
>
>> This problem could only occur on the system with 32 cores, when I
>> cut the cores to 16, this problem disappeared, so I think there is
>> some parallel problem when the 32 core set clock time together:
>
>> I try to reproduce the scene:
>>
>> 1.do_settimeofday64
>> 2.update tk time
>> 3.update base time offset
>> 4.update expires_next
>>
>> the 3 and 4 will be called in softirq, but the hrtimer_interrupt may
>> break the order and run before 3, I am not sure whether this could
>> make the problem, do we need to update base time and expires_next in
>> the hrtimer_interrupt?  maybe I miss something, thanks for any
>> suggestion.
>
> Base offset is updated in hrtimer_interrupt as
> well. hrtimer_update_base() does that. So that's not the problem.
>
> Can you apply the patch below and enable the hrtimer tracepoints and
> collect trace data across the point where the problem happens?
>
> Thanks,
>
> 	tglx
> ----
>
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 44d2cc0436f4..614f8d272cb0 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -575,8 +575,14 @@ static void timekeeping_update(struct timekeeper *tk, unsigned int action)
>   	update_fast_timekeeper(&tk->tkr_mono, &tk_fast_mono);
>   	update_fast_timekeeper(&tk->tkr_raw,  &tk_fast_raw);
>
> -	if (action & TK_CLOCK_WAS_SET)
> +	if (action & TK_CLOCK_WAS_SET) {
>   		tk->clock_was_set_seq++;
> +		trace_printk("TK: Seq: %u R: %lld B: %lld T: %lld\n",
> +			     tk->clock_was_set_seq,
> +			     tk->offs_real,
> +			     tk->offs_boot,
> +			     tk->offs_tai);
> +	}
>   	/*
>   	 * The mirroring of the data to the shadow-timekeeper needs
>   	 * to happen last here to ensure we don't over-write the
> @@ -1954,6 +1960,11 @@ ktime_t ktime_get_update_offsets_now(unsigned int *cwsseq, ktime_t *offs_real,
>   		base = ktime_add_ns(base, nsecs);
>
>   		if (*cwsseq != tk->clock_was_set_seq) {
> +			trace_printk("HR: Seq: %u R: %lld B: %lld T: %lld\n",
> +				     tk->clock_was_set_seq,
> +				     tk->offs_real,
> +				     tk->offs_boot,
> +				     tk->offs_tai);
>   			*cwsseq = tk->clock_was_set_seq;
>   			*offs_real = tk->offs_real;
>   			*offs_boot = tk->offs_boot;
>
> .
>

I don't try this patch yet.
But I found out when the cpu is stalling, basenow.tv64(7676664221321) is
bigger than ktime_get().tv64(7336008904750) in hrtimer_interrupt() and
the timer->_softexpires is 7336288000000. This makes it can not finish
the while loop until ktime_get().tv64 arrives basenow.tv64.

Is it correct that basenow bigger than ktime_get() ?

Thanks,
Yang