Re: HRTimer causing rtctest to fail

[Felipe Balbi <felipe.balbi@linux.intel.com>]

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Hi,

Thomas Gleixner <tglx@linutronix.de> writes:
> On Tue, 27 Feb 2018, Felipe Balbi wrote:
>> I'm facing an odd problem with v4.16-rc2 (also reproducible on v4.15
>> final) where rtctest fails sometimes which PIE coming too late with
>> frequencies >= 1024 Hz.
>> 
>> I've modified rtctest.c a bit so that it continues running even after
>> first failure just so I could get a glimpse of how many times it fails.
>> 
>> Here are the results:
>> 
>> Error 1024Hz i 17/20 diff 1089/976
>> 
>> Error 2048Hz i 2/20 diff 538/488
>> Error 2048Hz i 9/20 diff 558/488
>> Error 2048Hz i 16/20 diff 560/488
>> 
>> Error 4096Hz i 2/20 diff 305/244
>> Error 4096Hz i 7/20 diff 288/244
>> Error 4096Hz i 9/20 diff 285/244
>> Error 4096Hz i 11/20 diff 310/244
>> Error 4096Hz i 16/20 diff 284/244
>> Error 4096Hz i 20/20 diff 317/244
>> 
>> 
>> I added a few trace_printk() calls around rtc_dev_read() (and some other
>> functions) and captured the time for entry and exit of that function. I
>> noticed that HRTimer fires way too late and also way too early
>> sometimes. On the order of 100+ us.
>
> Power management, deep idle states?
>
>> For example with iterations 17 of 1024 Hz (seen above) and 18 (also of
>> 1024 Hz) I captured the following time stamps (all in uS):
>> 
>> |    Start |      End | Expected End | Diff |
>> |----------+----------+--------------+------|
>> | 35900626 | 35901711 |     35901603 | -108 | (too late)
>> | 35901826 | 35902628 |     35902803 |  174 | (too early)
>
> Too early? hrtimers never fire too early. The measurement in that test is
> bogus:
>
>                         gettimeofday(&start, NULL);
>                         /* This blocks */
>                         retval = read(fd, &data, sizeof(unsigned long));
>                         if (retval == -1) {
>                                 perror("read");
>                                 exit(errno);
>                         }
>                         gettimeofday(&end, NULL);
>
> It's measuring relative time between going to sleep and wakeup, so if an
> event comes late, then the next one will claim to be early which is
> nonsense because periodic mode runs on an absolute time line. 
>
> 		1	2       3       4	5	6	7
> Expected:	|	|	|	|	|	|	|
> Real:		 |	 |	   |	 |	 |	 |	 |
>
> 		 	           ^ late event
> So 2->3 would give you (period + latency) and 3->4 (period - latency) ....
>
> And if the task gets scheduled out before the taking the start time stamp
> then this is also giving bogus results.
>
> What the test really should figure out is the deviation of the absolute
> time line. That's hard because the RTC code starts the hrtimer with:
>
>      hrtimer_start(&rtc->pie_timer, period, HRTIMER_MODE_REL);
>
> which means relative to 'now'. So there is no good way for user space to
> determine the expected timeline.
>
>> I can't come up with a proper explanation as to why HRTimer is firing at
>> such wildly odd extremes. On most of the measurements, HRTimer executes
>> within 5 uS of scheduled time and I can't convince myself that
>> scheduling latency would get so high all of a sudden. Specially
>> considering I'm testing with a 4 core machine and there's nothing else
>> running on it. It's mostly idle.
>
> Enable the hrtimer and scheduling tracepoints. That should give you a hint
> what's going on.

Thanks, that does give me a lot more information. So here you go:

         rtctest-1348  [003] d..2   313.766141: hrtimer_start: hrtimer=00000000667ce595 function=rtc_pie_update_irq expires=313573983010 softexpires=313573983010 mode=REL
          <idle>-0     [003] d.h1   313.767189: hrtimer_expire_entry: hrtimer=00000000667ce595 function=rtc_pie_update_irq now=313574053764

We still have a 70754 nS deviation. After changing to absolute time,
the deviation remains:

          <idle>-0     [000] dNh2    29.303251: hrtimer_start: hrtimer=000000006858b496 function=rtc_pie_update_irq expires=28765551360 softexpires=28765551360 mode=ABS
          <idle>-0     [000] d.h1    29.303565: hrtimer_expire_entry: hrtimer=000000006858b496 function=rtc_pie_update_irq now=28765621916

I took a few measurements (76 to be exact) and plotted the deviation
from expected expiry time (now - expires from the tracepoint data) and
it's looking rather odd. PNG attached (all numbers in nS). Could this be
caused by FW stealing CPU behind Linux's back? I can take many more
measurements, if necessary. Just let me know.


cheers


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-- 
balbi

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